5th Gear Noise

2.0L FE, 5-speed manual.
Gears run fine, but when I get into 5th gear, the tranny whirs, almost like a metal-on-metal sound, but it's been running fine for almost a year (since I bought it), and still does. It's just that I know it's not supposed to make that sound, and I'd like to pin it down and make it go away.
The noise fades away when I release the accelerator, it's a low hum when I maintain one speed, and it gets loud when accelerating.
Strangely enough, 3rd gear makes a little bit of noise, as does 1st. All of the "forward" gears in the double-H. 2nd, 4th, and reverse sound just fine.

Any thoughts?

Sounds like a bearing inside of the transmission. But I bet you knew that already.

How many miles are on the vehicle?

You may be able to feather foot the truck for years before a transmission bearing fails completely. Any noise in the transmission amplifies through the case.

The reason the sound is like whirring is due to the way the gears are cut.

If you do a fluid change on the transmission, catch the fluid and run a magnet in the fluid you collect, if there isn't a magnet on the drain plug already. See how much metal shavings you catch on the magnet. That will provide a clue as to how much damage may be occurring. Little specks are normal, shavings are not.

Sorry... brainfart. '87 B2000, mileage is 279K

I don't know how long it was like this before I got it, but I've put almost 10,000 miles on it, and I know damn well I don't go easy on the pedal
Nor has it gotten any worse. It's just damn odd to me.

I'm pretty much a novice here, so if I drain the gearbox, what am I replacing it with, weight-wise?

80W-90

80W-90... thank you