Strange sound from exhaust...

I just got my DTLT headers and had them installed onto my 07 Tacoma.... the muffler shop cut off the front 2 cats and welded the flanges on the back two cats to run exhaust for me. i had them install a "X" pipe (magnaflow) between the 2 cats and 2 mufflers (flowmaster 44 series). ... since installation i have noticed a ratteling/ grinding noise that started after install, it chimes in at about 1200 RPMs? i know its the exhaust because it started directly after install and i took it back to the shop to see if they could figure out what was wrong but nothing.... any ideas on what the noise might be?????

 

Look and see if there is a flange or bolt close to the frame somewhere. I had a header flange that would only barely touch the frame when the car was in gear. I about went crazy trying to find the rattling.