Audi A1 - metalic clank when lifting foot off accelerator

[hutch100uk]

This is a weird one and I've had my car to 3 garages now with no luck. Summary of the issue below:

Makes tinny/metalic clanking noise
Only when I lift my foot off accelerator (foot is off clutch, doesn't do it if foot is on the clutch)
only in low gears
only at lower speeds (under 35)
doesnt do it when car is really cold (first drive of day)
usually starts to do it after about 10-15 mins of driving
usually disappears again after about 15-20 mins of driving
Cannot hear when car is stationery under any conditions

This has been going on for about 8 months. Had exhaust heat shield checked. Last garage thought it was coming from inside the exhaust so they took apart and put a camera in but nothing. Noise is STILL there.

I'm at a loss now and looking for any suggestions as its like finding a needle in a haystack now.

71 plate Audit A1 30 tfsi sportback - 66k miles

thanks