[chinners]

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.