Rattle on engine shut down

[Matt B]

I wonder if anyone has similar experiences or an idea... Every now and then when shutting off my engine I hear a rather loud an harsh rattle for a fraction of a second on the very last few revs.
There are no noises when firing her up or during operation (yet I sometimes hear a kind of knocking sound when engaging the clutch after shifting - though it's rather rare).
The rattle comes from underneath the car, I'd say clutch / bellhousing area. But I could not spot it any more precisely than that. It sounds a bit like a rattle can when you shake it very fast.
Car is an all original 20k mls car, runs and drives smooth and with no noticeable noises or play in the drive train.
Could it be a flywheel issue or noises from the gear rings? I'm using synchromesh fluid instead of engine oil in my gearbox.