Might be a drive shaft? My thinking is that when you lift off the clutch the drive shaft is being loaded

(and hence knocking), but when you dip the clutch it is unloaded and hence not knocking.

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.like I say it's a guess though- I'm sure an expert would have a better idea

Doubt it's the release bearing- I would expect that to make a noise when you push the clutch down

(ie when the bearing is being used)?

Probably worth jacking the car up and checking the wheel for play- might be a bearing, and the timing of the clutch is a coincidence.

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