I will hit a PM to John (scrming). I believe that the "rattle" you hear is a mid-tone baffle resonance that is an inherant aspect of the 4.0L SOHC. John had the MRT Aeroturbine and has had the majority of his work performed by MRT personally, so let's check with him. I trust his input.
In the meantime, I will link his website/blog and you can review his issues and some solutions he has come up with concerning his exhaust set-up.
Meh. This is why I come here. 'Tis an aspect of the job. Happy to help.
Unless there was a really bad fittament/installation issue, the MRT sounds like azs. No offense to MRT. Exhaust/Sound Tuning the SOHC 4.0L with a mid-range RPM reverberation is a hefty undertaking irregradless of the Mfg.
Is it a continuous rattle? or does it just rattle on the bumps?
Sorry to hear about the bumper... the fact the muffler tip was touching (or very very close to the bumper) make me think it's the mid-pipe issue... seem there is quite a bit of difference in the mid-pipe "rotation" from car to car...
What you can try is loosening up the mid-pipe... this is the pipe after the Y... try rotating it a bit.. ( i believe counter-clockwise..) What you are looking to do is give the mid-pipe more clearance over the axle... It also can help with the bumper clearance issue...
If that doesn't work with the bumper clearance you might want to try tweaking the hangers a bit...
I am going to try and rotate the mid pipe when I get home from work today.
As for the rattleing. It will only rattle when I stopped, then accelorate. It rattle's for a 2 or 3 seconds then stops. Any info you can give me to fix thix problem will be appreciated.
P.S. You have a sweet mustang !! Inside and out it looking good. I read you over your page. Good job with all that stuff.