2008 Prius - Squeek dealer can't figure out

Folks, I've had my 08 to the dealer 3 times over the past few weeks, got it back today after they had it for 4 days. A high pitched squeek that is not associated with braking -- only starts after the car has been running for 15 or 20 minutes, and gets progressively louder the longer we drive the car (especially on hot days) -- loud enough that after 30 minutes or so you can hear it doing 65 on the turnpike with the radio on and a/c blowing. Neither me or my wife can figure out where it's coming from (sounds like front end of the car, but not sure) and therefore can't figure out if it's coming from the steering, suspension, brakes, wheel bearings, whatever...and the dealer can't either. They've heard it, but have now stated that "it's a normal car noise" and not to worry about it. Doesn't get worse when going over bumps...just normal driving, accelerating, taking foot off the accelerator, or whatever will bring it on.

 

Makes no difference; A/C on or off...again, just a random (but persistent) squeek. I'll look for the notice about the seat repair that someone else suggested