Engine/drivetrain Flutter as highway Speeds

[jglopic]

So, you may recall that I nearly totalled my Expedition this past summer. I took out all the front bodywork, front passenger lower A-arm, rear passenger trailing arm, and scraped up a bunch of the undercarriage.

Now I am getting a flutter type sound when cruising at highway speeds. My first thought was exhaust leak (I hit the passenger exhaust where it comes out of the engine bay), but I haven't seen any other indications of one. I am stumped and don't want to start tearing the truck apart chasing something that isn't that big a problem, just a little annoying.

Any ideas?

Thanks,
Jay.