Clutch issue?

Hello all

I've got a recon 380 box and new clutch in my 90 with heavy duty fork. Thing is, every time I switch off my engine, with the clutch depressed or not, I can feel a single 'thump' in the clutch pedal just before the the engine stops. What is that? I can't recall feeling it in my other Landys.

Thanks for any pointers...