i think i had the same squeek when my system first got installed. the noise would happen at idle (barely) and i would actually here it when i closed the doors.
my mechanic and I traced it to a flange connection about about the 1/2 way point in the system (just before the muffler I think). using a pipe for leverage on the socket wrench, we tightened the bolts up as hard as we could and it went away.
now if the pipe going over the cross member would be so easy to fix....