i think it has to do with the lack of a harmonic balancer, and the vibration is partially "balanced enough" crank components... if you had all the rods and everything weight matched and had everything balanced well then it probably does that less..

Yeah, my flywheel was lightened by the way.

mine buzzes pretty good, i think it gets worse the lighter your flywheel is.

Might be engine mountings.

My A12GX also has a slight vibration when taking it up past 5500rpm.
The gear lever does'nt vibrate but when holding your hand the whole time on the gear lever you can feel that it has very slight vibration. I was informed that it is because it is not aftermarket balanced and normal for a stock factory engine.

For an A15, is it common for the engine to vibrate at about 5k rpm?.

Well, mine does. Vibration not in sense as though the engine's gonna die off due to low idle, but its a kind of high frequency vibration. Even the gear lever rattle's terribly.
This problem was not obvious before, but now, its quite bad. I even tried advancing & retarding the dizzy, it makes no difference.

Anythig to do with the mounting perhaps?...visually it looks fine for me...