A couple of funny noise related questions
 

Hi - I was wondering if any technical experts could tell me what's going on here and whether these 2 (unrelated) noises are normal - the car is a 2.5 V6 Conn SE auto:

1. When pulling away from cold, I hear a sort of mild 'thunk' come from the front. It only happens when cold and only the once. I did read about some sort of flap somewhere (manifold?) and was wondering if this is it opening. And what the hell does it do?! Or is it something else?
2. Growling - when the ATC is on, there is a low growl, noticeable at lower speeds. If ticking over, the growl is still there, but it gradually fades away until I can no longer hear it. Then as soon as I make off again it comes back. If I come to a stop it is still there, then fades again after 15-20 secs (rinse and repeat). This is not the fan, which I can hear cutting in at slow speeds - it is a separate issue but related to ac, because if I select ECON on ATC, the noise disappears. I can't imagine this noise is normal as it is noticeable - but perhaps it is normal? The ac by the way performs nice and cool - probably the coolest ac I have ever experienced in a car and lovely in the summer heat.

Just wondering if anyone can shed light on any of those. Is my car going to explode or are these 'character traits'? I did search for 'noise' but did not come up with anything that really matched these issues.

Many thanks, Mark

Can you elaborate? When does it 'cut in'. Does it also cut out when the aircon is running?

TC

The ac fan kicks in almost immediately when the car is at a standstill with ac on as I'd expect. It occasionally cuts out but only for a short period when still before reactivating. If the ac is off, it doesn't. When moving at speed, I'm pretty sure the fan does not operate.

I'm not too fussed about the fan - I'm sure it is OK and the noise I am hearing/talking about is not associated with the fan. It is not a fan noise - the fan noise sounds normal but this is more of a growling noise which is dependent on the RPM of the engine (it goes up and down).

Hi Mark,

First I am not a tech expert :)

But

1/ If you leave the A/c on Auto all the time (so that it comes on with the ignition switch) then the A/c compressor will kick in shortly after you start. This is probably the clunck. When the compressor kicks in the engine cooling fan should follow shortly after to cool the compressor. Another possible for the clunck.

2/ If your 'growl' noise could be described as a low whine type of noise then I would suggest the A/c compressor or its pully. I have had a whine from two Auto V6 75s much as you describe. Only happens with A/c in Auto mode and only audible at lower speeds or tickover. Noise rises a little as you increase engine speed. Dont switch the A/c on and no whine.. Switch the A/c to econ and it disappears.

Since Econ setting turns the A/c into a normal heating system and takes the compressor out of the loop I figured it must be A/ compressor or B/ pully as extra load put on it when compressor engaged.

Therefore 1 and 2 are connected.

My 2001 Saloon has had these symptoms for some 20k miles now and hasnt got any worse. Having said that I dont use the A/c great deal.

Do you know what, I think I have the same noises!

For Point 1: every ZT I've owned (one 1.8T and two diesels) does this, as had my old's 75s (two diesels). I think its got something to do with the PAS, never know why it only happens "once per day".

For Point 2: I've got the exact same noise (I'd call it a whine) that stops and starts with the AC being off or on, regardless of the engine being switched on or not. I suspect it's the AC compressor - I hope it's not on the way out!

Thanks for those comments - I do not believe the 'thunk' has anything to do with the fan starting because the fan normally has already started before I pull away and that's the only time when I hear the one-off slight thunk noise, from a cold start. I'm pretty sure as well that the compressor has already started (hence activating the fan at stationary speeds) so pretty sure the thunk has nothing to do with the ac - it can't have, because it happens when the ac is off as well.

As far as the 'growl/low whine sound is concerned, I was guessing at something to do with the pulleys or bearings (does the compressor speed depend on the engine speed - I would guess it does?) - I'm also wondering about the compressor clutch and whether this is a factor but have no real technical knowledge about how that works or what it does so it's all guess work really.

All I know is that the ac does work, very well, but that the second noise only happens when the ac is on and varies in pitch according to rpm - and that it gradually fades away after a short period if the engine is left ticking over - but it comes back as soon as the revs pick up again.

Regardless of all the above, everything seems to be functioning as it should. I'm just curious about what these noises mean just in case they are signs of impending doom. I've only had the car a few weeks and it's always been that way, and hasn't got worse or better.

Thunk when pulling away? Lower engine mount a possibility? Dave

Thunk when pulling away? Lower engine mount a possibility? Dave

I don't believe so - it only happens once, and only happens when driving off from cold. Doesn't happen if the engine is warm.