Idle Tapping - not engine speed related

[stevesey]

Was going to do some more investigation this weekend - but thought I'd post first to help narrow the field down.

I have an intermittent tapping sound at idle, sometimes it's there, other times it's not. Not related to engine temperature, not engine speed related (changing D->N produces a distinct change in engine noise/revs, tapping is absolutely steady volume/frequency). Open the drivers door and the perceived volume of the tapping reduces - suggesting to me its being transmitted though the bulkhead (and so probably a body mounted, rather and engine mouted component).

After searching the forums the only candiate I came up with was the purge control/regeneration valve on the inner wing behind the passenger headlight. Need to pop bonnet and grab hold of it when noise is present (sods law says it won't do it when parked in a sensible place of course).

Any other ideas?

 

[television]

It could be a sticky tappet, the purge valve ticks more than taps if that makes sense

 

[stevesey]

It could be a sticky tappet, the purge valve ticks more than taps if that makes sense

But that would be engine speed related - this most definately doesn't change if you flick between D and N (or blip the throttle in N/P).

 

[jberks]

I take it, it only taps when the engine is running?
Is it quiet when the ignition is on without the engine?

 

[stevesey]

Is it quiet when the ignition is on without the engine?

Yes (although sometimes the throttle stepper motor will buzz a bit)

 

[star]

heater duo valve?? put the heater on full heat and see if noise stops??

 

[stevesey]

Car was silent after pulling on the the drive tonight, sat and waited, noise started after about 1 minute. Still on footbrake in D, so put handbrake on, released bonnet and let footbrake of to go and investigate - noise stops as car settle on handbrake!! (Well the vacumm level does affect the valve). Waited a bit longer and it restarted, as expected was able to track noise to the purge control valve, grabbing hold of it you can feel the vibration.

Having searched here and the other forums there a couple of threads about noisy valves on C classes, but no-one has reported back that changing the valve did the trick (and a few "they all make that noise" comments).

 

[BlackC55]

Purge valve I suspect.

It is common for them to tick a bit and will do no harm.

 

[type49]

I've seen cars with a rubber washer under each mounting nut to hold it off the wing a bit. Might be worth trying but yes, they all tick & this is normal, although some are noisier than others.

 

[stevesey]

Just noticed that there was a cable tie from the brake pipe underneath that was wedged against the bottom of the valve, have pulled this out (wonder if that was transmitting noise into the body - hard to believe). Also will look at the mounting rubber to see in the isloation can be improved.

 

[stevesey]

Update:

Lifted the rubber mount off the front wing support so valve was just held by the pipes/hoses (noticed that the bottom pipe is very solidly mounted on the wing - so plenty of scope for noise transfer to the bodyshell and cabin). Valve was then canted over at a 30 degree angle.

Initially there was no change - however after a couple of days I realised I hadn't heard the noise at idle. So put rubber mount back over it's hook to see if the noise returned and aftera few more days it hasn't.

When you have the bonnet open you can still hear the valve ticking, but I don't think it's a loud as before and certainly the noise isn't getting transferred to the body/cabin any more.

Only explanation I can think of is that having the valve at a 30 degree angle has disloged some dirt form a key area inside and has quietened it down. Let's hope it stays like that!