Grinding Noise Under Passenger Seat

I own a 2003 mustang gt and have been hearing a nasty grinding, metal on metal noise underneath my car. Been told it wasn't my u-joints, rear end is good, tranny has no metal shavings, just replaced the tranny mount, and I have no idea what it could be so i need your help!!!!
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I assume you have an upgraded suspension. But just in case have you inspected it. on my 2000, I had a noise like that and during the restoration process I lowered it. When the tech removed the front springs one was completely broken the other had cracks in it.

Never hurts to inspect your u joints, but the last time I had a u joint go bad on an SN95 it made a clunk type noise, not a grinding noise.
Seems like the SN95s are quirky. Mine made noise, squeaked spit ground you name it. Upgraded the suspension with my restoration project and now I just have the normal but annoying interior trim squeaks and rattles which I will attack in due time.

Hell my 13 has a nosy driver door, hit the small bump in the road and the door chatters. Dealer Body shop has adjusted it 3 times. It quiets down then within a few hundred miles it get chatty again.