Pinion bearing issue??

Over the last few weeks I've started to hear a hum/whine that pretty clearly is coming from the rear of the truck under certain conditions. They are: driving on flat or down hill roads, speed between 45-60 mph. If I take my foot off of the gas pedal, I hear the hum/whine. If I press on the gas pedal even slightly or apply the brakes even slightly the noise goes away. I've done some research and it seems to point to the pinion bearing preload.
Anyone had this issue before? Any other things I should be looking at? I plan on double checking the fluid level this weekend.

 

I’d check the u joints first, that is a common noise to hear when they go bad, let off power and hear hum

 

I'll add those to the list.

 

In my experience its always been the diff carrier bearings going bad with that type of noise. If you go from coast to in the gas is there a bit of slack/clunk when transitioning? Thats also been a symptom with a carrier or pinion bearing going out. Could drive it around the block and open the diff filler and dip something into the oil inside and see if you see metallic sheen, obviously if a bearing is going out theres going to be some suspended metal and there will be some metallic shine to the oil as well

 

I really hadn't noticed any vibration, shakes, clunks until today. On the way home coming through town I tapped the gas a few times while coasting and noticed a small "clunk" from the rear end. Not sure if it was the driveshaft or axle. Also not sure if it was always there and I just didn't notice it until after I had read your response. Also not sure that I think it was bad enough to be considered abnormal for a sick up truck. With all that said, I'll add checking the fluid for shiny stuff to my list.

 

Checked the u joints and all is good.
Decided to buy a new drain pan and drain the oil for inspection. Not so good. The magnet had more on it than I would have expected after 30000 miles, much less the 7500 to 8000 since the last time I changed it. After letting the fluid settle for a bit I poured it into another container. Probably 50-75 specs in the bottom of the pan and that's in addition to several that flowed out of it. Guess its time to look for a rebuilt 3rd member somewhere

 

So...............
I ended up buying a new 3rd member from ECGS and got it installed the other weekend. Also ended up doing new brakes including wheel cylinders, axle tube seals and o-rings while I had it apart.
Took it for it's first drive and the noise was still there and it was 100% the same. Same sound, same speed, same volume, etc. I decided that rather than just throwing more parts at it I would take it to the dealer and let them diagnose it. They called this afternoon and said that the shop foreman and 2 "almost master" techs looked at it and all 3 think it's the 3rd member. I have a hard time believing this. If the noise was at all different, then maybe, but it's exactly the same. They said they also completely disconnected the rear driveshaft and drove it and the noise was gone. Am I completely out of line thinking that maybe it's a u-joint, even if the noise isn't a typical u-joint noise?
For now the plan is to leave it there until Wednesday when their full master tech gets back from vacation and let him look at it. I also had the idea to switch driveshafts between the 2 trucks this weekend and see if the sound follows the one.
Definitely open to any ideas/suggestions/input.

 

A little metal on the magnet is nothing to be alarmed about. That is why there is a magnet. Nor is 50 - 75 specks in the bottom of the oil drain pan. IF the differential was defective the oil would appear to be thick with metal flakes. You wouldn't be guessing.

 

So there was some of what you show in your pic, though not as bad. The diff was definitely pretty tired even if it wasn't "broken". I could spin the pinion flange with my pinky finger with zero effort.

 

In my experience with u-joints going bad, a shot or two of grease will quiet them down for a short time.

How much play is in your carrier bearing if you push up on the DS?

For reference this is what my rear diff drain plug looked like at 120k, with 30k on the oil

 

I did try to throw some grease in the ujoints but that didnt seem to quiet anything down.
If I remember correctly there was about 3/8-1/2 inch of play in the carrier bearing. I initially thought that was excessive, but it appeared to be about the same on my other truck so I decided it was normal.
When I took the drain plug out with 3k on the oil it was a mound at least 1/8 inch tall.

 

That amount of play seems reasonable

Wonder if pulling the rear DS out and taking it for a spin in 4Hi would help diagnose this? With no load on the rear diff

 

They did that at the dealer and the noise went away.
I called ECGS yesterday and they were leaning towards the carrier bearing.