W202 (1999 C200) Under-bonnet Whine/screech

[MGB]

Hi everyone..new to site..hope someone can offer me some advice re.above.

My C200 classic 5 speed auto with 29,000 miles has suddenly developed an unusual noise under the bonnet and I am concerned about safety or damage that may occur if I do not find out what it is.
Travelling along at any speed above 10 mph when engine is hot the noise starts. It does not happen on tick-over or on revving the engine. It happens whether the air-con is on or off. Everything is serviced and topped-up as far as I am aware and the car is in 'tip top condition'. I can only describe the noise as a low level screech which stops as I pull up to a junction. It reminds me of a very quiet fan belt sreeech but the car aacelerates fine and the fan belt is fine. The car is running great as usual apart from the noise. I have checked everything and just don't know what else to look at.Can anyone make a suggestion to what it might be.Many thanks in advance.

 

[paulcallender]

Brake squeal?

 

[MGB]

Mgb

Thanks for reply....The noise isn't affected by applying the brakes or not applying them and seems more under-bonnet than in the corners. It is more to do with engine speed. I feel I can dicount a brake problem but thanks for your time and interest.

 

[paulcallender]

......Travelling along at any speed above 10 mph when engine is hot the noise starts. It does not happen on tick-over or on revving the engine.........which stops as I pull up to a junction.

Just to clarify, does it only occur whilst the car is moving?

Its still possible that its brakes, they can squeal when applied and when not applied. It could also be the front hubs. As far as I'm aware, there is nothing within an automatic gearbox which could squeal. Anything from the torque converter forwards, spins with the motor so your stationery check can isolate this (unless its load-related). It is possible that the propshaft middle bearing is to blame here.

If its load related, put in drive and firmly press the brake with your left foot, then press the accelerator and 'stall' (let the revs rise until they no longer go above a certain point) the motor.

Okay, if its only happening when the car is in motion:

1. Drive up so, say, 30mph and switch the engine off. Does the noise go away?
2. Put the car onto a rolling road, eg typically found at MoT stations, and spool up the front pair of wheels and the rear pair of wheels. This could be done with the engine off, I think the rolling road has the feature to electrically drive the rollers.

Are you sure its front, not back, that the noise it coming from?

 

[MGB]

Hi..Thanks for your interest...Yes it is only while the car is in motion. It happens on load but is also evident on deceleration. It really does sound like a slipping belt but is a very low noise. I drove about 8 miles today to Southport and couldn't really here it to start with but on the journey home it became evident again. It only happens when the engine is warmed up.

 

[paulcallender]

OK possibly its a wheel bearing on the way out, or lack of/wrong type of grease in the front wheel hubs. As the car gets warmer, the brake and other moving parts warms up the hubs, and the grease reduces its viscosity. Then it becomes insufficient to adequately lubricate the hub(s) and a squeak develops.

I can't see how a drive belt issue could vary with temperature. It could vary with load applied by one of the ancillaries (alternator, power steering pump, air conditioning compressor) but I think you have eliminated variances here.

 

[MGB]

I will do some more checking/investigating and try to isolate where the noise is coming from. I really do look after the car and take a great deal of interest in the way it runs (and looks) and notice straight away when something changes. I have had the car just over two years and have had no other problems. I could just ignore this noise and indeed if the radio was on all the time I would never have noticed it but I would rather find out what it is. I will add a new thread when I have some more news....Many thanks for your ideas.

 

[woodturner-fran]

OK, you say it only happens when the engine is hot...

Could it be that when the viscous clutch on the fan has cut in that the serpentine belt (fan belt) is slipping?

Maybe check that the fan can spin freely and is not getting caught in anything. If memory serves me right, the play in the belt shouldn't be any more than say an inch.

let us know how you get on,

Fran

 

[MGB]

Fanbelt and fan both ok.Belt tension about half to three quarters of an inch. Today this intermittent noise was more noticable on deceleration than at any other time. Then I didn't hear it for a further 5 mins. Tried to grease and WD40 most moving parts today but no better.

 

[paulcallender]

Have you eliminated the engine from your enquiries (by switching it off whilst moving, or running the car on a rolling road)? Once you've done this and if the noise is still present, you'll have to eliminate/isolate other moving parts (brakes, hubs, transmission, driveshafts, differential, propshaft, etc). Also have you inspected the propshaft and wheel bearings (some dismantling will be required)?

 

[MGB]

hI pAUL...no have not done that yet for two reasons..1. not sure about switching off with car being an auto..will it affect anything safety wise? 2. The noise is not on long enough to be of any use. Only on for a few seconds intermittently (just like a short fan belt screech but not)

 

[paulcallender]

Yes, it will affect safety-critical items. Your steering lock will probably come on, you'll certainly lose power steering and servo from the brakes, after a few applications. Rolling an auto to a stop (after a short distance with the engine off) will NOT break it. This is why I suggested a rolling road (you mean you don't have one? Is your tool kit 5 spanners and a couple of screwdrivers LOL?)

No but seriously, you'd need to find a suitably quiet area to do this. Of course, you could raise the car on axle stands, run the engine and see if you can determine the noise. If you're on your own, this also has the advantage of being able to put a brick on the throttle, maybe run the car at 40-50 mph and then step out of it and have a poke around the wheels, etc. But you don't get the benefit of the suspension being loaded on the stands.

 

[tom7035]

I would plump for a suspect water (coolant) pump attempting to sieze, thus cousing slipping belt noise at these times. The heat issue is relevant too.
My tuppenceworth.

 

[MGB]

Thanks..TOM 7035...
Just done 16 miles today and there was no noise until the last mile when it occurred once as i reved slightly more going around a bend. I can't get away from the drve belt and have been looking at the tensioner. It seems quite easy to move to and fro. The deflection in the top of the belt is about three quarters of an inch which seems ok to me. I put a socket on the adjuster ont the tensioner pulley but can't tighten it . I dont know much about this mechanism only what's in the Haynes manual ! PS. I put some Barrs water pump lubricant/stop leak in rad.about 5 days ago incase water pump needed it !!

 

[MGB]

Success!!!!!!!!!!!!!

My problem has been solved (I Hope). The squeal/screech noise turned out to be a 'Hissing' noise and probably misled all the people who tried to help me on this site. I took the car to an ex-Mercedes mechanic for a drive and would the noise start...would it heck ! Anyway when we got back to the his garage he lifted the bonnet and asked me what the hissing noise was. I really hadn't noticed it and thought it was just the normal engine sound at tick-over. After a short tome he pressed on a rubber tube/gromet between the last two injectors on the right hand side of the engine (looking from the front) nearest the back. He pulled this one inch rubber tube off an unused accessory connection ? and hey presto no hissing.The rubber tube had a large split in it and was causing air to be sucked in causing the hissing noise. Apparently it covers a connection for something that is not fitted to my car. A new rubber tube was pushed on to the connector and I had a quiet journey home. This fault did not seem to affect the running of the car and my relief at this minor fault was enormous. I would appreciate any feedback on similar problems or any comments on this particular fault.Thanks to all who replied....Mike.

 

[tom7035]

Amazing how interpretations/descriptions of noises can differ so much and why it is so difficult sometimes to diagnose a fault from afar! - A far cry from a screaming water pump to a vacuum leak!

Glad you got it sorted.

 

[MGB]

UPDATE....Further info.

I THOUGHT I HAD ELIMINATED MY NOISE PROBLEM WITH THE RUBBER GROMET BUT REALISED SOON AFTER THAT THIS WAS JUST ANOTHER PROBLEM THAT CAME TO LIGHT WITH MY INVESTIGATIONS !!! THE RUBBER GROMET COVERING WHAT I HAVE BEEN TOLD WAS AN UNUSED PART ON THE TOP RIGHT SIDE REAR OF THE ENGINE WAS FOR THE DAYS BEFORE MY ELECTRIC GEARBOX. THIS NEW GROMET COST 74 PENCE FROM MERCEDES AND HAS STOPPED THE HISSING/AIR LEAK.
BUT......

THE FIRST NOISE I MENTIONED IN THIS THREAD IE SCREECHING, CAME BACK WORSE THAN EVER !!! I SWITCHED EVERYTHING OFF AND COASTED AND THE NOISE WAS STILL THERE UNTIL I CAME TO A STOP. I DECIDED IT WAS EITHER PROP.SHAFT OR WHEELS. I HAD ALL THE RUNNING GEAR CHECKED OVER AND NO FAULTS WERE FOUND. MY LAST CHANCE WAS THE WHEELS. I TOOK OFF THE REAR WHEELS AND DISCARDED THE LOCKING WHEEL NUTS. I WAS ALSO VERY CAREFUL TO LINE-UP THE WHEELS VERY CAREFULLY ESPECIALLY AT THE REAR AND MADE SURE I DID NOT OVERTIGHTEN THE WHEEL NUTS. COMPLETE SUCCESS!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

DOES ANYONE HAVE ANY VIEWS ON WHETHER MY SUCCESS WAS DUE TO :-
1. WERE THE REAR WHEELS SLIGHTLY OUT OF ALIGNMENT WHEN I LAST HAD THEM OFF SHORTLY BEFORE THE PROBLEM STARTED ?
2.DID THE LOCKING WHEELNUTS AFFECT THE HUBS/WHEELS IN ANY WAY AND CAUSE THE NOISE ?
3.DID I OVER-TIGHTEN THE WHEELNUTS ?

THANKS FOR READING THIS AND YOUR INTEREST AND COMMENTS. I AM SORRY THE ORIGINAL INFO. POINTED TO THE FRONT OF THE CAR IE ENGINE BUT IT IS QUITE AMAZING HOW NOISE TRAVELS......

mIKE.

 

[paulcallender]

Even if the wheels are badly fitted to the rear disc/drum, it doesn't affect the wheel alignment. It would affect how 'square on' the wheel is (on the hub), and thus the wheel would run ever so slightly from side-side, while driving. But the wheel alignment itself, which is the camber and toe in, would relate to the alignment of the axle itself and would be unaffected. Locking wheel nuts or overtightening would not affect this. Dirty wheels/hubs could affect how they sit together.

I suspect maybe during removal and refitting, you have 'jiggled' the rear discs or drums enough, to eliminate a brake squeal. Does this sound plausible?

 

[tom7035]

Maybe a detached pull-off spring for the brake shoes inside the drum, and fouling the inside of the drum?

 

[MGB]

Thanks to you both for your interest.......I suppose a brake/disc blip could have been the problem. It didn't sound like any brake sqeal I have heard before. It only happened at normal operating temperature and was getting worse until it stopped. It was a rear wheel area noise and just by taking them off, putting them back on minus the locking nuts and not overtightening the nuts, all is quiet and has been for over a week now. Mike