Odd noise

Hi all, wonder if anyone's come across this before. On a cold engine, pull away and when speed hits 15mph there's a double "clunk" from under the car, forward of me. It'll happen in any gear, as soon as speed reaches 15mph, and it won't do it again untill the next time the engine is started from cold. Other than that, everything else is fine. Gears light and positive, no baulking, clutch spot on, bites and releases 3/4 down pedal tavel consistently, brakes fine, no pull off centre, though they do sound loud compared to my mk4 Mondy I traded in. Mmm, any ideas? Cheers.