Odd pump/grumbling noise from engine bay when coming to a stop

[steamship]

Originally Posted by rory_pamphilon

My A8 is a 3.0Tdi QS 2007.

Just checked and the electric vacuum pump in the braking system only applies to the petrol engines (BBJ, ASN, BVJ and BSM).

Now this or may not be relevant, but might be worth checking for a perished pipe. The 3.0tdi also has a vacuum system on the EGR, but no idea how it operates. The attached image might help.

[rory_pamphilon]

Thanks,

I will have a look into this. Ive noticed zero issues really apart from the odd noise so not in a major hurry, its curiosity more than anything.

Will feedback if/when i know more.

Cheers

[mcs]

This will likely be your air suspension compressor. It turns off when the car comes to a stop. It also pumps out a jet of air through the outlet to purge the air dryer.

As the compressor ages and it’s piston ring gets worn, it will become noisier.

Mike

[rory_pamphilon]

Turns out after all it was my Crank Shaft Pulley. Apparently there is a rubber seal or vibration dampener in there somewhere? It had worn down and the pulley had slid out a bit.

It all came to a head when the power steering suddely stopped working and battery not charging indicator came on. Another 100m and the car started overheating so i stopped right away and got it towed to a garage.

Got the belt and pulley changed however the garage said the tensioner bolts looked like they might strip the thread if they tried to change and was warned it may be a big job to get them out if the thread does strip. They phoned me and let me make the call so i relectently decided to leave the tensioner as is as i didnt want to add any more onto the repair bill.

I hope i havent made a bad call, repair bill was already £1400 for pulley and belt plus rear discs & pads and one rear caliper.

[rory_pamphilon]

Hmmm so this noise has now returned despite all the work above.

Its coming from the left hand side of the engine bay, sounds like its just behind the glove box but could be further forward, difficult to tell as only occurs when in motion.

What is on the left hand side of the engine bay? Where is the AC compressor located?

Thanks

[steamship]

Originally Posted by rory_pamphilon

Where is the AC compressor located?

The attached image should help. The AC compressor is at the bottom right of the serpentine belt route.

[rory_pamphilon]

Originally Posted by steamship

The attached image should help. The AC compressor is at the bottom right of the serpentine belt route.

Thanks for this, does that make it a possible candidate then for odd noise coming from front left engine bay area?

I have tried driving around with AC on econ mode at times and its a little difficult to tell due to the noise not being too frequent but it does seem to abate when set like this however its not cut and dried, i.e. if i switch econ on when this noise is occuring it doesnt seem to kill it instantly. does the AC continue to run on perhaps for a little while when turning econ on?

[rory_pamphilon]

Originally Posted by mcs

This will likely be your air suspension compressor. It turns off when the car comes to a stop. It also pumps out a jet of air through the outlet to purge the air dryer.

As the compressor ages and it’s piston ring gets worn, it will become noisier.

Mike

So i just had the air suspension compressor replaced yesterday as it was taking a good while to raise the car in the morning.

Noise is still there lol! Driving my nuts.

[steamship]

Originally Posted by rory_pamphilon

air suspension compressor replaced yesterday... Noise is still there lol! Driving my nuts.

That's a real pain when you've gone to the hassle of replacing parts. Rereading the entire thread, did you ever pursue replacing the A/C compressor (not that I'm implying that that was the cause)? Just trying to eliminate possible options.

Is it possible to try and record the noise? I know it's not great, especially when the noise you want to record isn't that loud and the engine tends to drown most of it out. There's also the issue that unless it's a very unusual sound, most people won't pick up on it.

[ainarssems]

It's a bit of a long shot and hard to say without experiencing symptoms in person. Could it be climate control fan touching the casing?

I had kind of similar experience on Q7 after replacing climate control fan. Starting up, fan running at slow speed no noise; not sure if there was noise when driving and speed pushing more air through fan and making it spin faster and touch casing but me just not hearing because of other noises when driving. Or maybe it was braking forces shifting fan wheel just a bit so that it touches casing when braking but I could hear noise coming to standstill and it would stop after couple of seconds of being stationary. Taking it apart and reassembling making sure everything was seated correctly made it go away.