I just installed a Borla catback on my 05 GTP. I am getting pretty annoying resonating at about 1600-1700 rpm. The exhaust was used and came off an 04 GTP but is in great shape (0nly on for a few thousand miles). It is actually the catback for an 03 as Borla does not list a number for 04+. It does fit but uses it has an exta hanger bracket that is not on the 05. Anybody else have a resonance problem with a Borla exhaust? Any advice on a fix would be helpful. Thanks.