(shoulda responded sooner...)

Rotated them and the sound changed? Then you DID swap em around and you DID prove to yourself that it's the tires making the noise.

I'm gonna GUESS that the tires may have worn funny because of alignment or something? They're already worn funny, so you might not improve the noise until you replace the tires. But if you get the suspension all aligned & correct, you would avoid doing the same thing to the new tires.