squeak?

I have a 13 3500 with Aisin 6sp, while hauling about 15k in the mountains and pulling reasonably hard, the wife says she hears a "squeak" when the trans shifts to the next higher gear, I cant hear it but she does. I'm thinking serpentine belt, which looks good to me. Truck has 85k miles on the clock. Any other ideas on what it could be cause I'm stumped!
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I see you are still waiting an answer, I do not own a early 4th gen with the AISIN, but I would suspect the engine speed change is not abrupt enough to cause the accessory belt system to chirp. That being said, if the trans shifts abruptly, it could cause some interior plastic panels, bushing, almost anything to make a squeak. Our 2012 HO, (48RFE) has plenty of squeaks and growls in it but its a work truck. Perhaps have her drive and switch seats so you can listen for it?

 

Most likely she is hearing the turbo through the intake when it shifts. The intake is on her side, and she hears the turbo change pitch when it shifts.