Whistling / Grinding noise 2014 Suburban

[Jayjaved]

Hi, my 2014 suburban recently started making this grinding noise which sounded like it was coming from the bottom of the front passenger seat. The noise was not consistent, it occurs anywhere from 20 to 70 mph & has nothing to do with braking or acceleration. What's driving me nuts is that the sound does not happen all the time. The sound happens while slightly turning, driving straight, sometimes I don't hear it at all. I also don't hear it when it's raining out. After paying close attention I figured out the sound is coming from the bottom rear. The grinding noise starts out as a soft whistling noise which sounds like a wind leak from somewhere but then turns into a louder grinding noise from the bottom (similar to cutting deli meat using a slicer). At first I thought it was the rotor or wheel bearing then I had those checked out. Mechanic told me they were fine. I also had him check both differentials, axles, brakes, e brake, took it to the dealer & they recommended changing both calipers - so I did. Still hear the sound. Dealer told me the truck is mechanically sound & could not pinpoint where the sound was coming from. Took it to another local mechanic & the truck didn't make the noise, a few hours later I started hearing it again. Any ideas anyone?