LS4 Performance - Tapping from tailpipe

Hey all, I think I know what my problem is, but I figured I would run it by the pros....

2006 impala SS, when the car starts cold there is a noticable "tap" coming from the left tailpipe. Once the car warms up and has been running it stops (or at least enough that it isn't noticable). It is the stock exhaust, and I am thinking that the muffler is bad on that side. I talked to an exhaust guy who said that it may also be the resonator going bad as well.

Any thoughts/similar experiences?

I would think the muffler would be more likely than the resonator as it is before the mufflers and then splits to both, so any sound coming from it would come from both tail pipes.