Suspension/dog bone bearings? There's a few of them:
https://www.fowlersparts.co.uk/parts...-cushion-lever
The Abba stand takes all the weight off the suspension so the rear wheel hangs with the suspension at the bottom of its travel - you'd need to grab the rear wheel and try and lift (heave) to look for play. Based on my old Street Triple there will be a tiny amount of vertical travel. I spent some time looking for a knock that I could feel through the frame on my Triple - it was the catalytic converter shroud hitting an engine lug - on its prop stand there was half inch clearance (!) so I'd initially discounted it.
Could it be the pads rattling in the caliper? Gentle brake application would isolate that.