a-thrashin and a-clatterin!

[Major Rant]

Just a thought ... it could be the chain-guard.

If the feeble Aluminium bracket that holds the front end of the cheap-and-nasty plastic chain-guard gets bent inwards for any reason then the chain slaps against the guard and it sounds horrid...

Mine did it one dark night after I hit a deer - sounded like a washing machine full of 2p pieces.

It gets worse (intermittently) on bumpy roads and would tend to go away when the chain is tightened.