M104 Whiiiiiiiiiiiine

[Chazchuzzlewitt]

I've got an (I think engine-based) whine that starts dead-on 2000rpm and continues to around just under 3000rpm, which equates to 55-60mph in 4th gear. If I let the revs drop off to under 2000 it disappears. Isn't there if I slip into neutral at 60mph, so rev/engine speed dependant. Cruising at 70mph, at 3000prm, all is quiet again.

At first I thought might be bearing/gearbox/diff, but if I put it in neutral at 60mph (coasting downhill), it's silent again, but revving the engine while still in N past 2K I get the noise.

I've had the belt off and checked all accessories/pulleys etc, nothing untoward, belt & tensioner, alternator, idler pulley & fan pulley bracket all recent replacements (noise was there before all of these done), air/smog pump is removed.

Engine runs fine, no noise at idle, power good etc. Just that annoying whine.

The only thing I've read about that changes on the M104 at 2K is the VVT system, which is engaged at 2000rpm apparently...

Here's a sound recording of it, starts at around 5 secs in when I go over 2000 rpm, and you can hear it lessen as the revs go nearer 3k, then drop out again under 2k...

https://soundcloud.com/user-468757519/whiiiine

 

[chardF]

I'm guessing you've ruled out the diff? Whine at a steady speed, disappears if accelerating or decelerating.

Can you get it to whine when stationary? or is it only under load @ speed.

 

[Chazchuzzlewitt]

Pretty sure it's not the Diff as it still whines at exactly 2K in Neutral with the engine revved (i.e. no power to the prop/driveshaft) , and having had a Diff go out on me before, it definitely comes from the front of the car.

Forgot to add, ATF level is correct and recent fluid/filter change, could the Torque Converter make this noise though?

I still need to test it properly stationary, haven't had a chance yet to find a helper with the car warmed up and have a good listen...

 

[chardF]

You can't get it whine while stationary?

 

[Chazchuzzlewitt]

Yeah it does it when stationary, I just haven't had a chance yet to get a helper rev it at exact RPMs while I listen properly with my head in the engine bay!

I've listened to all the top accessories -Idler/Fan pulley bracket/Water Pump/Tensioner/PAS-SLS pump at idle with a mechanic's stethoscope (i.e a big screwdriver), and no nasty noises... alternator is almost new. When I had the belt off to check all the pulleys the A/C compressor pulley bearing was very slightly noisy when spun but no play in the pulley.

 

[chardF]

Try it with the belt off.

 

[Chazchuzzlewitt]

Try it with the belt off.

Yeah I think that'll be my next thing to try.

Last time I had a noise like this it was on my old Diesel, turned out to be bad vacuum pump bearings.

 

[hotrodder]

As Chard, running the engine briefly with the belt off would be the next thing i'd try... a rear wheel bearing on mine started to get noisy before there was any signs of the bearing being on last legs when spinning a wheel and feeling for roughness etc, could be something similar going on with one of the belt driven pumps etc

Details on the cam advance/retard mechanism can be found in section 5 here http://www.w124performance.com/service/Index/104index.html specifically 05-2160. While it advances the intake cam timing at ~2k rpm it doesn't (or shouldn't) retard it again until ~5k. It'd throw a fault code(?) but could unplug the cam adjuster for a brief test much like unplugging a MAF to see if the engine runs differently

Auto boxes can howl sometimes but AIUI it's usually when the pump it's on its last knockings and like bearing noises usually gets worse/louder as the bearing rpm increases?

Longshot but i wonder if the resonance valve in the intake manifold could cause it? It's supposed to alter the 'effective' length of the manifold and is switched soley on rpm... when it's working it's responsible for the almost 2 stroke like step in the torque curve just before 4k rpm. AIUI it's more common for them to break or seize and cause a rough idle though

 

[Chazchuzzlewitt]

Just an update in case anyone is reading this in the future- the whine turned out to be the viscous fan coupling. It was a Chinese made one fitted by a previous owner. Fitted a genuine BEHR and all is quiet again!