Squeaking out of the rear pipes - yipes!

Car: '72 Duster.
Engine: 360 crate.
Exhaust system: TTIs, Flowmasters, new pipes from headers to mufflers (all new from engine back to mufflers, and pipes behind mufflers older but in like new shape).

This is odd. Since getting the car's exhaust changed the car is squeaking. It's not a metallic sound like metal to metal, but it sounds just like a mouse is squeaking actually, and loud enough to barely hear over the exhaust note.

I've been all over the exhaust and can not see any contact areas that are improper.

Does anyone have any ideas, or should I just go back and look for metal on metal? Thanks in advance,