how i no this sounds stupid (it happened on my m8s golf when he changed his susspnsion) i undid his wheels nuts on N/S front and we could not get the top mount loose so he left the front shocks untill the day after and we put allthe center caps back onto his bbs, forgetting id undone the nuts and all way home car was nocking, fine on idle and slow speeds, as he acclelerated it was louder etc..... so check the nuts and all the botls around the susspension.
alternativly it could be the exhaust has come loose, i no mine has jus cba putting rubbers back on lol, other than that i cant really think of anything else except when driving at high speeds does you steering wheel move or pull in any way???