Vibration into car on cold start 1.6 tdi black edition

Hi I recently got a 1.6 tdi black edition 2014 with 10k on it. Main dealership had to fit a new clutch at 7K, single mass FW on car (ClHA engine) recently on morning start there is a vibration noise that starts can only be heard and felt inside car with clutch in or out.. Dealer had a look and nothing found. They had large plastic undertray off to remove bell house for clutch. Looking at posts on here and further google checks im thinking this is a loose fuel pipe?? Underneath When i rev the car at idle a few times the vibration goes away and not heard when driving and only comes when at idle and not really heard at operating temp?. Any views