Could be a few things...
One of your spherical mounts might be broken on the passenger side causing the sag. Remove the two 10mm bolts located on each side of the shaker. Gently lift up on the shaker to remove it from the spherical mounts. If one is missing you'll want to replace it.
If all 4 of them are fine, it could be that the shaker itself is just loose or the isolators were removed by a PO. Mine was rattling shortly after I got it and found out the isolators were shot and allowing the shaker to rattle, bounce excessively.
I replaced the worn out isolators with a new nylon spacer and a couple rubber grommets to quiet it down. Did it a in June of '14 and it has been fine so far.
Years ago there was a TSB issued for shaker rattle. The factory isolators that the front of the shaker rests on were junk and needed to be replaced. The link below has some good info.
Mach 1 shaker scoop squeaks & rattles....HELP!!!! | Mustang Forums at StangNet
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