I didnt over bore the block no, it was just a stock 81mm bore and pistons.

I stupidly did not check my piston to wall clearance. Can i check this by removing the sump/oil tray etc? Just by feeling for excess play?

Its really hard to tell exactly were its coming from, especially as it only under-load.
It doesnt get any quieter or louder as it warms up, thats why i doubt its piston slap, because forged piston expand more when hot dont they?

If a rod main had like 0.04 clearance instead of the 0.02 would that cause a slight knock underload, or would it be obvious (get worse with miles quickly and knock on or off load?)