It sounds to me like you are describing the packing in the silencer.
This needs to be replaced on a regular basis. Buy some new packing from the bike shop. Wrap around the screen, hold it in place with a couple strips of tape (just needs to hold long enough to get it back into the can) and reassemble.
When you get to the bike shop you will find several different brands and types of packing, all claiming to be superior. Pick one, any one. Next year pick a different one and compare. If you like one over the other, stick with it, or try a third the next year.
Rod