Originally Posted by mxd via PM

check tranny, drain the fuild and look for excess metal or black fluid/green fluid... green is metal, black is heat, if you have normal gear fluid color with minute micro metal shavings its normal wear, if you have black stuff or large metal chunks its on its way out, and if its green with metal chunks, its already toast and you are lucky you have motion..

from my experience if your tranny is goin, you can salvage any miles left by keeping it out of stress (under torque or horses or high rpms) until you get a replacement..

if it was a main input bearing you will get bad bearing noise in neutral, if its output it will cause vibrations/harsh bearing noise underload .. if its only gear specific it might b internal gearbox, if its a go between gear all gears will have problems except 4th.

also in my experience, after changing the fluid in a bad tranny it acclerates its demise i have not yet had a bad 5sp or 4sp take fresh lube and survive much longer,, metal shavings in tranny work like sawdust in crankcase LOL

i am not a certified mechanic, here is to grains of salt . .

good luck to you

Originally Posted by mxd via PM

ok, if u changed ur fluid and it was black and stinky that is normal too, but way over due. in neutral it should sound like skateboard wheels on smooth pavement, if you hear gravel and smooth pavement then it might be your input shaft bearing.. that noise is consistant and goes rhythmic and with rpm, this is with your ear to the shifter. volume of the rolling bearing sound is relative to wear, basically if it isnt a rolling click in neutral its most likely not main input shaft. Out put shaft OR differential input bearing on the rear end have the same symptoms in noise and vibration underload and if one is bad enough it may damage the other due to the flow of vibration along the drivetrain. sometimes rearend seals do not leak and the bearing is going or toast, so a leaky seal is not expected. the mazda drivetrain is balanced, any play in low play places can lead to stuff like that, I am assuming that your wheels are balanced to at least 70mph and your car is not all corroded, and the suspension is good condition.. something like that could be as lame as water in a tire, so it is important that you take the car off the ground and feel around to make sure drive shafts and everything is tight.

with high speed vibration if you have totally elminated tires and the propeller shaft (take it in they can test its balance at any well stocked machine shop), then it has to b rear diff or tranny gearbox.. it might just be the propeller shaft.. and nothing to do with the rest of the expensive stuff! that is were i start to spend money first (driveshaft)..

im not a mechanic, but i break a lot of stuff, and make autopart store managers wealthy and annoyed.

cheapest to most expensive:
tires
propeller shaft (can b expensive if u go carbon)
rear diff
transmission

Originally Posted by a_drift

If the pilot bearing is out, I think you're referring it to the "input shaft bearing," then you will here a lot of noise mainly when you push in the clutch and the input shaft is spinning freely with the weight of the clutch on it. But even with it going out it will not give you vibration when at highspeeds. I know because I've had em go out on me like 3 times, I've had some bad luck, but then again I beat on the car on the track a lot. Now my best idea would be the u-joints, just wiggle the driveshaft up/down and side to side while trying to keep one side of the u-joint still. You can just jack up the car and do this. It also could be something with your differential. Check around and let us know what you see.