Hot start issue!! bang on turnover , only when hot??

[Romperstoper]

Hi, I'm new to the site . I've got a vw Passat 2.0 tdi highline on a 2009 plate . I've owned the car a few months now. I've noticed when the car is up to temp(at the 90c) the car makes a bang noise on start up . It only does this when hot, on a cold start its fine . This does not effect the way the car runs or drives (hot or cold ). and since I've had it I've had my local vw centre give it the once over cost £96. They couldn't find any problems with it , they plugged it in and couldn't find any faults codes. Tbh they weren't very helpful with the hot start bang , they did re-program for the mpg scandal . All they said was it could be the dmf (dual mass flywheel) but it would have to get worse b4 they would no what the problem was. I've been told if it was the dmf it would make the noise hot or cold? I'm not sure .... if anybody has any idea plz let me no i'd really appreciate the help... Thanks in advance