I have an 18 inch Delkevic on my CBR250 (2011)

the stainless steel has become all burnt (not really a concern to me)

But I would guess that it means that the packing inside has also become burnt?

The sound has changed from deep bass to a higher whine-ier annoying pitch.

Not so much a concern for me.

But a loud buzz/rattle happens now at certain RPM’s. (Low RPM’s on deceleration, to be exact)

I tightened up all the clamps, even put on a new rubber ring on the main muffler clamp.

Had the bike serviced, new chain slider as the old one was torn to bits.

The baffle is tighter than a duck’s arse.

Still there.

any ideas?