Been hearing somewhat of a grinding noise on my 99 Dakota 4x4 5.2 78,000 miles auto. Originally thought it was coming from the differential. My father in law helped me narrow it down that the noise is coming from the front of the driveshaft where it connects to the trans. The noise starts around 10mph to 30mph then i don't hear it anymore. Also, when you press the gas you can hear somewhat of a hissing sound under the cab. When you let off the gas it goes away for a few secs then under motor deceleration it comes back. The hissing noise happens more with faster speeds from 30 mph and higher.

It shifts fine, goes into gear fine. 4x4 works great. No popping noise other than a clunk going into reverse. Crawled underneath it and the driveline has no play. I checked all wheel bearings and they seem fine. Any ideas??

 

is the hissing possibly vacuum?
I have a buzzing noise in m 97 sounds lik a tiny gear spinning when im slowing. I dot know what It is?

 

The hissing could be an exhaust leak. Start the truck and crawl under it. Have someone block off the exhaust a bit, not all the way, and see if you hear the noise.

Not sure on the grinding. Pull the driveshaft and check the u-joints to see if they're binding.

 

I would still check the differential. Jack up the back of the truck, disconnect the driveshaft from the pinion yoke, and turn the yoke by hand. That's the best way to feel if the pinion bearings are rough.

Noises in the rear axle have a way of traveling through the driveshaft and appearing to com from other places.

 

Would like to know if you figured this out finally. I have the same issue on a 99 Dak 4x4 318cid 5spd. Getting ready to yank driveshaft and start the whole process of diagnosing.
Thanks -