Manual transmission - bearing whining noise??!

I hear a light (very faint) whining noise from the transmission at 40MPH(My truck reads 44.5K miles as it sits). It was fine until 40K miles and it started this noise recently. I went to inspect the oil level and found out that Toyota in their infinite wisdom decided to not put a dip stick. Anyway, the oil level seems fine. I changed transmission to synthetic oil (Redline MT 90) at around 35K miles.
My truck: 44.5K miles, 6 Speed manual, 6 cylinder.

Any thoughts?

 

Does it make the same noise in several gears at 40mph or just one gear? Any change in sound if switch to 4wd?

My 6 speed is noisy in 6th. Never noticed until I put in the core shifter. Put the stock rubber shifter back in can still faintly hear it. Pretty sure it was always there but now that I've identified it I can always hear it.

 

Does having the clutch in make a difference? My throw out bearing was replaced under warranty about that many miles.

 

Very few manual transmissions ever had a dipstick. I can't even think of any off the top of my head.

I've had/seen a couple of manual transaxle vehicles with a dipstick, but even that's been a while. Maybe they still exist?

The only way to check your fluid level is very similar to checking the diffs. Have it level F/R and L/R, then pull the fill plug. If fluid trickles out just a bit, or you can touch it with your little finger it's full.

 

I would say 6th gear, mostly.

 

Well you have already changed the fluid and put in the good stuff (MT90) so I would say monitor for awhile and make sure you don't tow anything in 6th. If its a higher frequently wine then likely a bearing slowly getting worn. If a fast tick-tick-tick then likely a chipped tooth and will progress quickly and need attention.

 

By the way, 40 mph is too low for 6th unless you're slowing down. I usually don't shift to 6th untill at least 50 mph. Lugging can be hard on clutches and transmission