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Jeep 2009 Grand Cherokee 4x4 Laredo left rear noise at highway speeds

I have a 2009 Jeep Grand Cherokee 4x4 Laredo. For approximately the past 7 months there has been a noise coming from the left rear of the vehicle at speeds above approximately 60 mph. The sound is not metallic in nature, it sounds more like a seat belt flapping against something. The frequency is approximately 2 Hz and is very consistent at highway speeds. The driver can seldom hear it apparently due to interior acoustics, but the someone in the passenger seat or the back seat hears it plainly, over a period of time it is very annoying. In addition, if you touch the left rear interior side panel (in the cargo area) it seems like you can feel a small vibration that matches the sound. I have had the vehicle inspected by Jeep, they find nothing. I have had the left rear interior panel removed to see if there was a problem with the sound system cable bundle that runs through there, but it was OK. We thought it might be the metal gas tank door vibrating in the wind stream, so we taped the door down, but the sound was not affected. Does anyone have any ideas on what the source of this sound might be? Thank you very much.

Rotate the tires and see if the sound moves with any of them. While rotating, inspect for any abnormal tread wear.

The only time I have had a similar noise complaint on this vehicle I had to track down it turned out to be a rear axle bearing. I like the above suggestion of starting with a tire rotate and balance to see if the noise changes.

Inspect roof rack if applicable.

Now that rattlegas mentions it, I did have a loose roof rack cross-member once that was making the oddest of noises at highway speeds. It drove me nuts since I spend all of my time looking at tires and bearings and sway bar links and …

My first suspect would be something like a tire out of round. Follow @cigroller 's advice to rule that out. One time I had a noise like this on my Corolla that only occurred at higher speeds, seemed to be coming from the passenger rear direction, and it turned out to be the rear side window (that rolls up a down) wasn’t sealing tightly and it caused some kind of air-movement resonance.

The tires have been rotated, there was no unusual wear pattern. There is no roof rack cross member. The sound remains in the left rear at highway speeds.
I plan to have the rear bearings inspected.
Thank you.

Do you notice any unusual sound at low speeds while turning, like turning in a parking lot? Especially when turning to the right?