You probably have a little wear on the engine bearings. I wouldn't worry about it much but put a little lucas oil treatment in it(10% of capacity) so the bearings will have a little lube remaining until the oil pressure comes up. My 83 1100 did that and I put a spin on filter adapter on it so it wouldn't take as long for the oil pressure to build. That lessened it from a few seconds to just a couple of knocks.
try 20/50 oil....what you have now?.....turn it over with the plugs out and see if any fuel come sprayin out...after sitting overnight or longer....if so one or more float needles are not closing...just a hunch!!