Beko WMB81445LW washer rumbling noise

Customer reported rumbling noise when drum started to agitate which faded away after a few minutes.

I checked on spin with a full load – no probs
Checked on a wash cycle – no probs
Checked tub and drum assy for debris – none found
Almost given up but then checked on a rinse cycle and it started rumbling. Removed the belt and restarted and it continued to rumble briefly.

Couldn’t feel play in rotor, and end float not excessive. This is an inverter driven motor and I’m wondering whether it’s a faulty motor or the supply from the inverter is not always consistent. Anyone else come across similar?