I just had a cat back muffler job done and a month later I noticed that sound again. The sound that seems to be a bad muffler. The muffler doesn't look damaged, no holes or anything. I will take it back to the shop, but could it be the cat? Or could it need a tune up?

 

If you think you have an exhaust leak something may have come loose on the new system, let them check it out for you.

 

Check the heat shilds on your CATS and Y pipe. They sound awful when loose.

 

check heat shields, ect. for anything rubbing, rattling. Make sure nothing is loose.