My first thought was driveshaft, but the differential is probably a stronger contender. The transmission is also a consideration, but less likely.
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Lots of great replies here:
@SUBURBAN5 : I personally did not do the seals, they were done by a friend of mine. He said he checked the bearings, but no, we did not check the pinion other than a cursory visual inspection. This makes me very curious...
@Nad_Yvalhosert : What you're describing is exactly what I'm hearing. Sound is loudest at 50-70kph, and I hear it with steady throttle, or soft pedal increase, and it disappears entirely when i let off the gas to slow down slightly. It sounds like I have to seriously inspect that rear differential. We did replace the seals, and when we did we noticed that the previous owner had really farm fixed the rear diff. In fact, he had sheared 2 bolts on the diff housing, that were still stuck inside the bolt holes. We had to get new bolts, and it was a pain in the figurative butt to get those old sheared bolts out... Problems in the diff would not surprise me.
@Pinger :I don't hear any difference in noise when applying the brake, regardless of if I apply without any throttle, or if i slightly apply while maintaining throttle. When you say the axle is a possibility, are you suggesting the axle itself? I'd mount my action cam up under there and take it for a drive, and upload the vid for you to listen to, but there's rather a snow storm that blew through here last night, so I have to wait for weather to smarten up, lol! If it were the axle, am I risking any sort of catastrophic failure to drive it around?
@Pinger :I don't hear any difference in noise when applying the brake, regardless of if I apply without any throttle, or if i slightly apply while maintaining throttle. When you say the axle is a possibility, are you suggesting the axle itself? I'd mount my action cam up under there and take it for a drive, and upload the vid for you to listen to, but there's rather a snow storm that blew through here last night, so I have to wait for weather to smarten up, lol! If it were the axle, am I risking any sort of catastrophic failure to drive it around?