I too had a horrific rattle upon deceleration (brand new M3W). Thought maybe it was the canister but foam did not fix anything.
Then I looked at the rear sprocket. The belt was snug against the outer flange, and when hand turning the wheel backwards you could see how the belt rode up on the flange. Also there was about 1.5-1.75 inch play in the belt. This picture is after I adjusted the wheel to center the belt and took out the excessive slack. But you can see the damage to the outer flange on the sprocket.
