When you say you set the valves "loose" how loose ?
They should be .003"/.005" and I would adjust to the .003"
It's possible a baffle broke loose inside the muffler which if moving around randomly could restrict exhaust flow which would cause a sound like you described.
Take the muffler back off shake it around and see if it rattles, Tap it a little and shake again. Try blowing air through it to see how much flow you have.
Originally Posted by piston slapper
You never learn anything until you admit you don't know it all...
"Why go out preserved when you can go out beat up, worn out, sliding in sideways screaming, Holy Sh!t What A Ride" !!!