Strange sound at high rpm

Hello guys,

I hope you can enlighten me with this as I have not a clue to what it could be.
If I get the bike (1989 TownMate T80) into the high rpm range a strange knock/tang can be heard. It's not in sync with the engine rpm as it's frequency is quite less, like 2 to 3 times/s. If I keep holding the gas or accelerates, it continues after dissapears for some seconds or maybe not. This happens usually in: 2nd gear at around 23 mph, in 3rd gear at about 33...
I'we also seen that all the above applies in general -- but on level surfaces. If I go downhill the speeds at which it happen increase
Also, if I grab the throttle gently it will appear later, than if I accelerate full gas

please, any ideas?!
I have the impression that is not engine related.

Maybe loose baffles in silencer

Hmm...could be! But I have no other exhaust to test
I tried something since I saw your post... if I take it slow (rev it gently it will never clank or produce any sound
it only happens if I gas it hard (100% throttle)

Looks like silencer. This may also result in loss of performance. Best to buy a new silencer they can be bought on line for less than £50.