My wife was painfully translating and article at a book shop with A15 engines rebuilt on top of a tyre outdoors out in the wind and this was for 10,000rpm A gatta series competition.
A series comped cars only and the one in the article wasnt the only one using a stock A15 crank with spiffed up rods, pistons, head and the rest but the crank was left standard.

They are bullet proof engines and unfortunately we cant help much with that info.

Can you post a vid up using photobucket.com?

are your rods and pistons all -/+ 1g in weight from each other?

So many things could be causing this issue

i am punching in the dark here... could it be you installed your piston backward? i did that once lol.

I've had A15's that buzz no matter what you do. The only cure appears to be a harmonic balancer from a company here in Sydney called Ross Balancers. A friend of mine in Newcastle has done this & it cured his harmonics problem, which was due to a knife-edged crank. It could also be a broken valve spring, which will buzz & transmit noise through the car.