I have been hearing a humming noise on my 98 Pathfinder (234k miles) that has a few facets to it. While under 40MPH it seems audible weather I have my foot on gas pedal or not, sounds like it's coming from the rear of my car. Around 45-60MPH it is HIGHLY noticeable while I have foot on gas pedal, and immediately goes away when I let off. If I push and let go several times in a row while at that speed it starts and stops with each step on/let off. Once I get over 60MPH it seems to be at such a high pitch that it "blends in" and is much less noticeable.

After Googling thee symptoms the most stated reason I read is rear differential pinion seals/bushings. I read forums from other types of cars saying to immediately stop driving, and others that say it is common and has lasted thousands of miles without worsening.

Does anyone have experience with this issue? If not taken care of soon is catastrophe on it's way? Is this an easy diagnosis for a professional mechanic? Is the cost to repair probably going to be an insane amount?

As always I appreciate you taking the time to help me with all my car issues!

Definitely sounds like the rear diff is the issue. My first order of business would be a rear diff fluid change. At the very least, you can see if there's any 'glitter' in the fluid.

Beyond that, you will need to get under the truck and push up on the driveshaft near the diff (hard). See if it flexes up and down. If so, you've got a bushing issue.

More likely, there's some "lash" in the differential (caused by wear) that, under load, causes the teeth to mesh together improperly, causing the noise. It's not a STOP EVERYTHING issue, but it won't get better on its own. At 234k miles, it's probably past-due for repair.

My first order of business would be a rear diff fluid change.

I changed the rear diff fluid about 8k miles ago. It was dark and more than likely overdue for a change when I did it. I don't recall seeing any "glitter" to the old oil but wasn't looking for it. One goof I may have made at the suggestion of the auto parts clerk was using non LSD gear oil but adding the Lube Gard LSD supplement to it. Was that not the right choice? I've never had any problems up until now with the humming.

I'll check the driveshaft tomorrow, thanks for that tip.

Is this something that is inevitable or could my use of non LSD gear oil+LSD additive been the cause?

Even if it was, it doesn't matter now - the wear pattern on the teeth is likely off a bit from the misalignment, and with that kind of mileage, it's actually not surprising. I'd guess the dark material was clutchpack wear residue.

If you're on a budget, you can probably find a lower-mileage 98-04 rear end from a Xterra / Frontier / Pathy (double-check with the experts here to make sure they're compatible) and swap it out, or simply go have the diff rebuilt (might not be terribly expensive).