Shake, rattle and roll.

[Ib-Denmark]

Hello,
This winter I bought a new piston for my G80S 1957 at the club spares scheme. The piston is +60.
After honing the barrel to a clearence at 0.10mm (0.004 inch) it was assembled. But I have a very sharp, metallic rattle which sounds to come from the barrel? The sound is only at low revolutions. Not much difference at cold or hot engine. After 600 miles I opened the engine, but everything seems to be fine. The engine is still rattling!
Any ideas? Is this common? Could it be something else? Ignition fault perhaps?

Regards
Ib Vestergaard, Denmark