loud engine noise on start up.

[stuspla]

audi a3 sportback 1.9 tdi.
a few days ago i went out in the car and returned to it 10 mins later.on starting the car it made a loud noise from the engine like metal was vibrating so i turned it of quickly.the car then started perfectly and has run good since.until today i nipped in the shop and it did it again when started,so i turned it off then on and has run good.could this be caused by not letting the engine lights go out properly before starting IE.a fuel problem or if this was a major problem would it do it constantly????
the car has just had new clutch and flywheel!

[NHN]

Urm if its had a new clutch flywheel recently & this issue wasnt there before, then stands to reason its related to the work, given its metallic in origin, I'd get it back to them to be checked as could be doing further damage.