It isn't unusual for the M47 to make gear noise, which can change as the gears wear.

But if you're getting a high pitched whine when you're going around 100kmh, that's a different story, and it will be related to 1,2,and 3 making similar but lower pitched noises when accelerating.

I've played with M47 transmissions and their fluids a LOT.

Personally, I think that 10w30 is miserable for the M47. You'll bang gears and have trouble getting synchros to engage. A few people have had luck with that fluid, but I have never found it to be worth a dime.

If anything, Redline MTL is a great choice. Since you have noise coming from the box already, put it in (yes, overfill it) and change it every 20,000 miles. For $10US a quart, you can't argue with that kind of preventative maintenance.

Synthetic might accentuate leaks, however. Type F trans fluid won't leak much, but the synth might show up more. Common spots are the output shaft and the gear selector seal. THe input shaft isn't usually as bad of a leaker because it's aligned via the clutch and pilot bearing and doesn't work the seal as hard.

If not, change the transmission fluid to Type F, and then change every 10-15k. It is SO easy to change, there's no reason not to. Personally Type F seems to shift just fine. Again, I've changed and experimented with almost every acceptable lube available. Currently my newest M47 (that I just replaced) has Type F. The 10w30 didn't cut it there either.

One last thing. The 5-speed box seems to dislike being removed and reinstalled. I don't know why, I just know that.

Finally, an M46 will need a different clutch. I'm not even sure if the flywheel is the same for the M46 either, but that's just a "what if".