I have the same problem as you, and I found what is causing the noise.
It is the plastic liners in the rear quarter panels.
Reach up under the rear quarter panels and tap on the plastic liners.


Pete...

98.5 SE Sport Duratec ATX (P66L)
Black Clearcoat on Greystone
DOB 11/21/1997
DOP 12/25/1997
PIAA 510 Super White Driving Lights
SVT CAT-BACK EXHAUST (No Resonator)
DiabloSport Chip (LPQ4) - Engineered To Raise Hell
Too many other mods to list...
Sounds sweet, runs quick...
Crazy Horse Dyno @ 121.4 WHP/129.6 TQ