2007 GT mustang exhaust help needed!
 

I recently got my catalytic converters removed and it sounds amazing except when my car's RPMs get high or idling when it's pretty cold. During these times it will almost sound like there's rattling in the muffler area, which I had flowmasters put on it a while back and in my opinion I think the mufflers can't handle as much airflow as the removed cats creates. There's no possibilities of anything being lose or broken.
Could someone help me out with what could fix this problem? I can post any pictures/videos if necessary.

What kind of midpipe do you have with the flowmasters? Stock midpipe with hollowed out cats or cats removed? Aftermarket O/R X or O/R H?

I had a similar experience when I first installed an O/R X with race bullets (perhaps a little different in my case given that I didn't have chambered mufflers, and don't know what type of midpipe you have). Sounded great, but had that ticking/rattling at idle when cold and trumpeted considerably right around 3000 RPMs.

A set of high-flow cats or perhaps bullet style muffler used as a resonator might slightly tame your exhaust just enough to stop the rattling at idle and high RPM without sacrificing a lot of volume or killing the exhaust note you like...

As far as I know everything is stock except what I've done to it myself in the past year. Midpipe and all of it. The cats were removed and just a piece of piping was put in it's place. I'm considering a muffler delete pipe to replace the Flowmasters to increase the air flow

A while back Flowmasters sometimes would break a baffle and cause a rattling sound. That could be your problem.