W212 350d 2016 engine/gearbox noise

[Kirkmeister]

Hi,
I posted recently about an unusual noise coming from the drivers side of the engine bay, thought to be a gearbox related issue though there are no gearbox problems.
I've attached an audio file recorded from the front drivers wheel arch.
The noise only develops after an hours running, and only when you move from Park or Neutral into D or R. It stops after you break 15mph.
If you can play the audio file it sounds as if someone is bouncing on a chair to make it squeak. The car is not moving though and the steering wheel is straight.

There has been a suggestion it could be connected to the eco boost pump as part of the stop/start system which is why it kicks in at low speed.
I cant afford to have the gearbox stripped to inspect (which has been advised by my independent garage) if its not a gearbox issue.

I'd be grateful for anyone that can run the file for any thoughts. The car runs faultlessly (96k miles) but I'm sure something is failing.

Many regards
David K

 

[philnewmerc]

It sounds like something or nothing.
If there is no change in the way the car drives and the gearbox is behaving itself, there is no way on earth a sane person would strip the gearbox. Just drive it and enjoy it until something breaks, then you'll know what's broken and can get it fixed.

 

[Tonygw]

I had a listen, doesn't sound gearbox related. Could be a pump or pulley making the noise. When you put it into D or R the engine is under a small load and this changes the engine running slightly as its loaded.

Have you had a stethoscope onto the alternator and aux pulleys to eliminate them, or event the water pump? There is a lot of moving parts and when they are all loaded they can make some noise.

But I do tend to agree with philnewmerc, the sound doesn't sound sinister. I would carry on driving, monitor it to se if it gets any worse. My car creaks a bit under load and sometimes I can hear the alternator trying to recharge all the battery while running all the cars electronics. The noises on mine come and go, nothing is a worry. Although mine is the om651 C250 on 130K.

 

[Neiltodd]

Pulley as above

 

[E55BOF]

It's not the gearbox, and I doubt it's a pulley; more like a pump (do you have air suspension?), or the EGR/engine breather system. Oh, and find another independent garage...

LIke philnewmerc, I doubt it's serious.

 

[Kirkmeister]

Good evening, Firstly my apologies for not replying earlier to your suggestions, I'd been away.

Thanks for the concensus it's not the gearbox, that's reassuring.
I'd struggle to 'live with it' as i have a lot of mechanical sympathy and if it's making a noise, something is in conflict.
I've got a real soft spot for my W212 as it is one of the last of it's generation and plan many more miles with her.

Thanks for your suggestions though. I'll run them past my technician, hopefully he'll not give me the usual reply "bloody forums...when I were a lad........"

 

[Kirkmeister]

Good Afternoon,
The source and correction of this unusual noise is now completed. A technician at my local dealership (Inchcape Cheshire Oaks) took the time to take the car out for an extended run so the noise could be replicated.
His diagnosis was a faulty auxiliary oil pump which backs up the main gearbox oil pump to help with the 'stop/go' feature amongst others. Now replaced, that annoying 'squeak has gone.
Happy days and here's to a good few more miles.