Hmm just went out for a blast and tried to hear where the sound was coming from... it sounds like a boomy exhaust that resonates at certain engines revs above 5.5k rpm... will have it checked out. I am planning to replace the clutch, suspension, cables for the clutch, accelerator and brake, steering rack and anything else that needs doing... I want that 'new car feel' that I had when I had my other low mileage Mk2 16v. The engine seems fine.