Hey Al==Check the tail pipe and see if it is mashed closed. Good luck Joe
A hole in the muffler would cause a knocking sound in your muffler. A loose exhaust pipe can also cause a knocking noise.
Damage. Hole. Perished gasket. Incorrectly fitted. Corrosion.
loose baffle
No. It will cause a problem with both back pressure and noise.
Remove the muffler
Yes, because without one, the noise of the engine or motor would be too loud, and the muffler serves a purpose of applying back pressure for compression.
Something to check would be if there is rust in the break-drums that would need to be cleaned. Another possibility would be the strap around the muffler. Check to make sure it isn't broken. If it is, the muffler may be hitting part of the frame, hence the noise. Other than that, I have no idea.
That's very likely, yes. Get it checked - a "flapping" exhauast system won't last long, but if you tend to it immediately, it may be an easy, permanent fix.
What would cause a grinding noise after a tire is replaced and the brakes are not worn?
It is likely that the rat is sick so I would take it to the vet.
Unlikely. A more probable cause would be that there's something wrong with the exhaust/muffler. A bad hub would have other symptoms, like poor steering, pulling when braking/acccelerating, vibration etc before you'd begin noticing the noise.
Maybe the muffler is bad.