Good thing no one described how to disassemble the bottom end!
"I've had many worries in my life- most of which never happened!" Mark Twain
Glad it's an easy fix
I think I've found the source. Not certain, but it sounds like a baffle in the muffler has come loose. Pressing on the side of the muffler almost silences it. Pretty certain now that it's nothing serious. Thanks for your help.