I had two muffler header pipes break due to vibration on my US40 (OS 70 FS). I finally braced the muffler to help prevent the third one from breaking.

I've also noticed that when the wing is off the plane if you simply talk loud close to it, it resonates from your voice. Also, if you slightly bump the wing it makes a loud drum sound. Maybe this has something to do with the vibration that is "burning" the dowels.

Keith