someone else on here reported the same thing. I'll see if I can find the thread and see if they came up with an answer. I remember posting on that thread because I told them about a dream a had when I first got flowmasters on my f150, i dreamed my exhaust stopped being loud and it turned into a nightmare!
found it..but nothing useful.
I would go back to the shop that installed it and have them take a listen and a look. Couldn't hurt anything