So the crossmember thing wasn't the source of my particular issue.
After hours of frustration I finally traced the noise to the inside of the rear fascia.
After taking it off
I removed the white styrofoam filler piece inside the bumper and wrapped up a few of the wire harness/connectors with electric tape and stuffed a towel in there for "insulation" to further hold everything still.
Put the bumper back on and went for a ride.. No more vibrations.
I got on the highway and hit some speed and the rear fascia still seems solidly in place.. My guess is that styrofoam piece is just in there to "save" the bumper if someone hits you.
Nothing but clean mean exhaust note now.
Worth noting I took out the tools and styrofoam from the trunk also, and wrapped any plastic wire harness/connectors in electric tape in the trunk also. And also tighten up (don't overdue it and sheer bolts) any nuts in the trunk and inside the bumper.