Strange Noise ...
 

Hi all

Any help would be appreciated, I have a strange intermittent engine noise, almost like a worn bearing of some sort?

It doesn't appear to be speed dependent as in doesn't increase in intensity or speed and can happen both at idle and when under load, just seems to stop and start.

I also have what must be a noisy air conditioning compressor or the like as I have a separate (very loud) whirring noise that only happens when the air conditioning is running, goes off if I switch to ECON or turn the heater off all together.

Any likely culprits of the other noise though? :Confused:

The AC noise is usually the electric radiator fan.

The other one is hard to discern from your description. Is a screech, a rumble, a howl, a rattle?

It sounds like more of a rumble/grinding sound, definitely sounds like it’s coming from the engine area while driving, seems to happen more when the engine is warm and when under load/accelerating but doesn’t seem to increase with intensity with speed if that makes sense.

Very intermittent though, just sounds for a few seconds at a time?

Could be the hydro fan bearing. They tend to rumble, although usually its constant when its failing. Ditto alternator bearings.

Can you hear it while looking in the engine bay?

Sounds like resonant vibration induced from somewhere into something. Whilst that's not very helpful in one way it might help in thinking about what's causing the noise. You say that it can occur on idle and under acceleration so perhaps as the cylinder firing/exhaust passes through various rpm it may cause resonance at various harmonics. e.g. at idle of 850rpm the engine is firing 4 cylinders per revolution (so far as i understand), therefore the firing rate per second is 56.6 p/sec. If that rpm causes induced vibration in something (e.g. exhaust components/ broken baffles etc), multiples of the same frequency (harmonics) can do the same which is perhaps why it occurs at different speeds/rpms. I might of course be talking out of my own exhaust and and its caused by a loose scewdriver rolling around in the boot but i'm trying to think out of the box.....:ROFL:

I'll get my coat.....

A few years back my mate bought an S3 from the main Audi dealership. About a year old. Chuffed to bits he was.

First time on the motorway there was an audible rattle. Went back to the dealer, who found nothing. Same thing happened again, so it went back again...& again...& again. Four times I think.

Eventually the culprit was found...one of the mud guard bolts was loose.

Took a lot of Audi technician hours to find that!

Ive only heard it while inside the car driving, tried to replicate while on the drive and listening to the engine but ticks over fine, no unusual noises, seems to happen more frequently when under load and when the car has done a couple of mile?

I might see if I can try and record the noise from inside the corner and post it on here, as its hard to describe

Interesting you say that, the car has just been in for a service and they picked up a few items in need of repair, 2 of them being broken exhaust mounts, could this be the issue?
They also picked up various suspension bushes, all of which are being replaced next week