Intermittent Whistle at Speed

I have a 2011 DCB OR with approx. 153k miles and I'm searching for the source of 'reedy' sounding whistle that I'm hearing from, what I think is the right, rear passenger area of the truck.

I get the sound while driving over about 45-50 mph and it comes and goes.

I've had the wheel bearings, u-joints, and carrier bearing checked and those are all okay. I've also poked around under the hood and don't believe the noise is idler or tensioner pulley related. Truck isn't throwing an trouble codes either. I also serviced the rear differential to try and rule that out and the oil looked fine with minimal metal shavings/goo on the plug.

I've also tried the usual door seal whistling noise fixes, i.e., silicone conditioning, drip tubing insertion, additional weather stripping, etc. The truck currently has an aftermarket windshield so I don't think it's that known issue either.

I'm starting to suspect it may be coming from the pressure vent behind the rear seat, but before I remove the seats to access the back cab wall, I thought I'd ask this esteemed group of experts for their two cents. Thanks!

 

Does it happen only when there is a strong cross wind?

 

It's not wind dependent (it's not the infamous kazoo noise).

 

Prolly already tried it, but I'll still through it out there. Make sure the windows are ALL the way up. My RC makes a horrid whistle noise if the window isn't all the way up, but still into the seal.

 

Do you have intact windshield cowl covers?
Also, did you check the seals on the doors and cab seals in front of the doors?
When I got my truck these items were in rough shape/missing. Before replacing them, there was some higher-pitched wind noise for sure.

 

So here's an update to close the loop. Finally broke down and took the truck to a shop, and after a day of diagnosis, they identified the problem as the seal on the back window. Got that replace, the whistle is gone, and the truck is more quiet than I remember it being.