If, as you say, the issue is not with the hitch or its installation, I would get it looked at by a good muffler shop. It is at least somewhat likely an interference issue, something too close. The muffler shop should be able to quickly figure it out and they will have the parts to properly fix it. Please share the fix with us when you get to it. My 2" Curt on my '07 is perfect - no issues at all. Tows great, too.