1991 Mazda Miata, 5-Speed... Transmission makes funny noise.

Did you check the gear oil level? This may require the car to be up on a lift as most manuals provide no easy way to check. Also, make sure you have the correct grade of gear oil. Too thick oil will cause the gearbox to make noise and feel a bit odd when you shift.

I didn't, and don't know how, but I'll ask the mechanic I'm taking it to today to change it to whatever oil is appropriate. Would you think that's a good first step?(providing the mechanic doesn't immediately, convincingly tell me what the problem is)?

I honestly wouldn't know where else to start. I had this happen to my BMW once, the lube place I went to convinced me to change the gear oil, and they put in the wrong stuff. I noticed it immediately when I drove away. They corrected the problem and gave me a free oil change coupon.

Does the sound stop when you step on the clutch or if you put tension on the shifter? Some cars used to make a whirring noise if the throw-out bearing was worn. Putting tension in the shifter (and hence, the bearing) would cause it to stop as long as the pressure was applied.

That's all I can think of right off hand.

Bearing, I remember that word, isn't it a synonym for money needed?

It won't be the oil or the diff because it stops in fourth. It is probably the thrust washers and/or the main shaft bearings. You will need a major transmission overhaul to fix it. Someone has been thrashing that car Bill. Shouldn't cost too much to fix though.

I took it to Fort Lauderdale tonight to see the Passion of the Christ... (way overrated) and did some thrashing of my own. 30 miles each way at about 100 mph. The area by the shifter seemed to get a little warm but I don't know if that's unusual either. Sound is most prevalent when I'm accelerating... none at all with clutch in, in neutral or fourth gear. Pressure on the shifter in various directions had no effect on it.
Mechanic I took it to today was a putts. He drove it, said it was going to be expensive to fix, but then said it wouldn't hurt at all to drive it as is. That didn't make much sense to me; so I made an appointment to get an inspection done at Aamco transmission tomorrow (they seem to be the experts in the yellow pages). This is a very cheap car. For another one or two thousand dollars I can buy a much better specimen and I'm wondering if I'm just asking for trouble being a cheap-skate or if I've actually found a bargain worth fixing.
Adrian; what do you mean by "shouldn't cost too much to fix though"? Also... is it dangerous to drive it as is? I've put 150 miles on it (hard miles) and it doesn't seem to have changed at all. If it's going to be expensive; I think I'd rather just by another car and get rid of this one. It has new top, new tires, new brakes and good paint and its a blast to drive but like I said; around here Miatas are a dime a dozen. Thanks everyone for your advice!

RUN AWAY FROM SCAAMCO AS FAST AS YOU CAN! THEY ARE RIPOFF ARTISTS!

I think you are right about Aamco. I just paid them to tell what I already knew. They estimate $1,000 but don't really know what the problem is. I guess I'll try the dealership... perhaps their mechanics have encountered this before.

Absolutely stay away from Aamco, Bill.

Your best bet is to find a good, reputable transmission shop. But if you can't, at least get it diagnosed by the Mazda dealer so you know what the problem is.

What's worse case? You're driving a cheap Miata you like that makes a noise. If it's too expensive to fix, but still driveable, just drive the thing until it breaks.

I hear ya Slappy... At the rate I'm going; I'll probably go through $2,000 worth of my time before the car breaks anyway. Its not like I don't have a cellphone or live in the sticks (plenty of cabs). Seems silly, but I kinda like this car.

Try the Charlie Brown solution:

Turn the radio up louder!

I also have a mazda miada 1991 and to makes noises when i am shifting the gears. You can hear the gears clattle. Any solutions. It drives fine and no problems. other than that.