When the engine heats up, your "tight" joint becomes "loose".

Get the engine going and HOT.

Shut it down, and promptly clean it off quickly so it's still warm.

Pull off the back and clean it. Apply some high temp gasket seal compont to the joint, put everything back together and apply Locktite Blue to the screw/nut holding the muffler in place.

Remember that the SCREW does all of the work, not the nut ( if I recall correctly on this muffler ).... the nut is merely a retainer.

Screw it down tight, but be careful not to strip anything... and let dry.