I had installed flowmasters and now I am getting a sound from my rear end that sounds like a bad spring in a matress, It only happens after the car warms up and I have driven it for a few miles. Any ideas on this one?

 

I assume you did the exhaust yourself on jack stands. If that is the case you may want to check your rear springs to make sure they did not move off their perches when you jacked up the car & or when it was resting on the stands.

Or maybe the hangers are not right a causing the noise.

Strange?

 

recheck your fasteners make sure everything is tight.