2000 blazer LT 4x4. While driving I hear a noise from underneath sounds like it's coming from the back of the truck somewhere. I just got the vehicle from my father in law and he just had the tcase replaced and his mechanic told him that its the front wheel driveshaft spinning. But I don't know if I buy that, I don't think the shaft should be spinning when in 2wd. But the noise definitely sounds like something that is rotating and making a humming/rubbing/whining noise. I'm wondering if it could be some part of the driveshaft or differential possibly?

 

If you have the 4-button (Auto-4wd) optioned truck, then your front driveshaft will spin even when in 2wd. This is due to the preload in the viscous coupling for the front output. It cannot fully disconnect the drive chain.

The front driveshaft for the 4-button transfer case has a CV at the rear joint where it connects to the transfer case. This joint is easily damaged if removed improperly which could be the case when the transfer case was replaced. You can grab the front driveshaft and check for play. Any play can result in a noise and/or vibration feeling coming from under the driver seat.