I have a feeling it's those clear plastic sheets that are between the door panel and door shell.

Two Challengers and one Cuda that I have make that freakin' annoying shrill whistle. They have been played with at some point in their lives. I have one other survivor condition E body and another fully restored one and neither of them make the whistle. So out of 5 E-bodies, three do it and two don't.

I might get time to fool with them this summer. If it kills me, I'll find it.