2005 Dodge Ram 1500 4x4 transfer case

I have a 05 Ram with 75,000 miles that is making some strange noises. It sounds like it’s coming from straight under the cab and it’s a high pitched grinding sound. It only happens after being driven for 20+ minutes around 55mph, but it doesn’t do it until I slow down in a town getting loudest when I coast through corners. It won’t do it for my mechanic, just me. Any thoughts? It sounds like hell when it’s making noise but the truck doesn’t lurch or feel like the transmission is slipping. I can feel a vibration through my feet. Been going on for about 2 weeks now, off and on. I think it’s my transfer case based on where the sound is coming from but my mechanic tells me that a transfer case will usually just quit transferring power to the wheels or vibrate so bad that I won’t even want to drive the truck.