I recently installed a Borla catback on my 08 GT, and sometimes at idle or low speed the car makes a loud vibrating sound from underneath. Of course it only happens while I'm on the road and never in my driveway. So I can't locate where part of the exhaust is touching. Has anyone had a similar problem after installing exhaust?

 

Bang around on the exhaust with a rubber mallet and you can usually hear where it's vibrating.