just purchased (my95) import wrx wagon, seems all good apart from an occasional strange, short, sharp, quite loud vibrating sound when lifting clutch pedal and engaging gear at high revs. it seems to come from low down and towards the back of the car. is this simply the vibration of things in the doors or something? or am i about to reget my perchase? pls help, lost for answers, any ideas welcome. cheers.

ps does the dump valve contain any perishables? mine seems to flap rather than hiss.