I have noticed a thumping non rhythmic noise at speeds 30-60mph. Comes from right rear. I have rotated tires and it is stilll there. Have 100k on it , original struts, neew tires last spring. Can the muffler create that sound if it's got a loose pipe/baffle inside it ? I have recently looked under car , grabbed the muffler, and it feels tight,despite looking a rusty tin can on the outside layer. Also moved wheel back and forth and it feels tight.

 

my muffler on my old gxe had a layer of rust on top of it covering the outside area side to side and it was connected to the muffler basically in one small space, so it was like flap of rust, therefore everytime I hit a bump over 35 it would make this rythmic noise and go up and down hitting the muffler. I would inspect the top of the muffler to see if you have this flap of rust that I had.