Vibration diagnosis

Hello all,

I'm trying to diagnose a vibration, which although often very faint does get quite loud on occasion, but it's largely random. Car is an Octavia II 1.6 FSI.

Always occurs when decelerating through 1800rpm, gear is irrelevant. Stops when clutch is depressed. Haven't managed to replicate it at any other speed or rpm.

Any ideas? I thought clutch release bearing but it's happening in the opposite sense.

Thanks