I fitted polyurethane diff mounts and now I hate my car
 

Fitted some polyurethane diff mounts as I have heard these fix some of the sloppyness in the 6 speed shift.

THE GOOD

The shift feel is really nice now, it has completely fixed the issue I had where I couldn't get the box into gears smoothly/easily.

THE BAD

The whole car is 10x noisier (and it was pretty damn noisy before). I can hear lots of extra road noise and the exhaust is somehow noisier (or just more engine noises transferring to the body). Between 90-100km/hr the car has this REALLY bad rattle accompanied by a low frequency hum - basically it sounds like someone has put a subwoofer up to your ears and cranked it.

Do you think it could be my tailshaft not balanced or something? It's no where near as bad as this in my friend's MSM with the same bushes.

Yeah, I put MMR mounts in my car and didn't even notice a difference. Is your exhaust hitting your diff?

That's good to know, I know my exhaust hits the brace at times when I'm doing crazy stuff but I don't think it is resting on anything. I'll investigate!

Something isn't right, you should not be able to notice a difference.

Thinking of this more, did you torque everything to spec when you reassembled? If you're getting a harmonic buzz, that's a likely culprit.

Something was put on backwards/upside down.

I don't know about not being able to notice a difference. I installed polyurathane bushings on my wife's 97 after replaceing the diff housing and it is a lot louder at 50-70 mph than before. I am 99% sure it is just diff whine and not other issues since I have replaced the driveshaft and axles with no change in the noise. I am at the point that I will be swapping in OEM rubber bushings when I get a chance to. I have been driving her miata as my DD for a while now while I am doing the 1.8 swap in my 91, most of my drive is on country roads right at 65mph where it is at its loudest and it drives me nuts. I tried to attach a sound file but it will not let me. If you are getting an exhaust or heat shield interference noise you should be able to tell it apart from the constant humm that you get from the gear whine at the right speeds. I wish I had researched these diff bushings more before I got them because I bought two sets to change out both my miata's, but now I don't even know if I want them on my miata and it is loud as hell already (exhaust).

I definitely noticed a difference. Specifically around 3-4000 rpm and especially on decel. Loud humming, the whole car vibrates.