very loud grinding in 8th gear

I'm taking a Prostar with some cosmetic damage to another terminal to switch into whatever will be my permanent assigned vehicle.

At first I thought the loud noise I was hearing was related to the front end damage - like a fan rubbing on a bent shroud. But once I slowed in a construction zone, I realized the noise stopped completely when I dropped down a gear.

Through trial and error, I can determine that the noise is only present while in 8th gear, which is the top gear in this transmission.

The noise is intermittent at a constant speed - it will grind for several seconds, go away for several seconds, etc..
Under power/accellerating, the noise seems to go away. Once at a sustained speed, it comes right back. Backing off the throttle reduces the strength of the vibration/grinding, but it is more constant.

And by noise, I mean a sound like grinding trying to shift into a gear, only about 3x as strong, and LOUD. I'm sure the traffic beside me can hear it.

I spoke to our OTR department, and they feel it will be fine to drive as long as it doesn't worsen or intensify. If nothing else, I can limp to a terminal in 7th if need be.

But I'm just curious what you all think this might be. Bad synchro on one gear? Bearing getting ready to let go? Low fluid level?