Exhaust note change??

[moto9]

Here's a weird issue I have come across a few times and I wanted to see if anyone else has experienced it.
When I'm working the bike pretty hard, like slow moving thru a rocky riverbed, all of the sudden the exhaust note will get substantially louder and the bike won't run as well.
Almost like some of the packing got blown out???
It seems if I rev the bike wide open and cover the silencer it'll quiet back down.
It did it with the FMF turbine core, the stealth and now it did it with the Leo vince.

 

[goose202]

2 stroke? Sticky power valve?

 

[GMP]

My first guess as well. Nice thing is you can pull the PV parts without removing the cylinder.

 

[moto9]

My first guess as well. Nice thing is you can pull the PV parts without removing the cylinder.

That could be it, I'll have to check the PV.