hi guys, need a bit of help with my exhaust. currently im running a twin 2.5inch stainless setup, which splits into a left and right rear muffler giving me quad tips. ive been having problems with one of the rear mufflers, when starting up from cold it sounds a little bit rattley, and when planting your foot and hitting the higher rev range it makes a wierd whistling sort of noise like the inside of the muffler is loose or the tips not on properly. any ideas on what i should be looking for or is the muffler cactus?

ive also found some exhaust pieces for sale, consisting of a fg gt exhaust (the non varex type one) and an fg f6 one. from visual inspection it seems the front half of the f6 unit will match up with the rear half of the gt one, giving me a ford factory look. if my current muffler is kaput, would it be cheaper to go down this path than try to replace the stuffed unit and match it in to the muffler on the other side? would running this setup be quieter than my current aftermarket setup? also as my car is a non turbo 6, am i right in the idea that running the f6/gt setup (from memory it has a twin 2 inch at the start, then goes to 3 inch through the middle muffler, then splits to a 2inch left, and a 2.5inch right pipe) would be better for performance? i already have extractors and hi flow cat, but the car seems a bit doughy off the line. cant afford a whole new system and the rattle/whistle is really starting to get to me.....

This is my current exhaust
https://www.youtube.com/watch?v=68M5zXITglQ
Pacemaker extractors, 100 cell cat, manta 3inch exhaust with hotdogs and quad tip

I get a strange whistle rattling sound sometimes as well, especially at high rev ranges.
Have you noticed it happens when the car hasn't been running long?
I was told by a mate who is a mechanic that it is the cat, because it's cold it causes that sound and given that the exhaust is pretty big it amplifies that sound a lot and causes it to sound like one of my mufflers is banging round under the car

Possibly the same thing going with yours?
I'd recommend actually getting under the car and inspecting for any holes, loose parts or damage.
You might be able to fix the problem with something as simple as a spot weld, extra bracket or it might not even be an issue and it's just a resonance sound caused from the cat being cold on start up