MK4 Golf 2.0 GTI - Strange noise when accelerating

Hi all

I have a 2000 2.0 Golf GTi which I have owned for about 6 weeks.

When I accelerate, there is a strange gurgling sound from the passenger footwell. It happens in all gears in all speeds, but only at the very end of the throttles's movements (only when I toe it, basically).

It kind of sounds like a pump which has air in it, but I'm not sure.

I've asked two garages about it (they both say they can hear it but everything appears fine) I've also recently had a full service, MOT, new cambelt & tensioners, new water pump and new catalytic converter.

If anyone can give me a clue to what this noise is (its not that loud, but definitely noticeable) I'd be very happy! I like my car but I've spent around a grand on it since I bought it and I don't want it to be a money pit!

Cheers all

Chris.