Hey everyone, I posted this exact thread in the suspension/brakes category as well, but I figured if I put it the drivetrain section maybe I can get more help. Here's the deal below.....

I originally put a set of H-tech springs in my 04' WRX sedan w/factory struts but I got a vibration that feels like its coming from throughout the chasis when I hit about 60mph through 65mph. You can feel it in the seat a bit, but you hear it humming more than anything. Its only there while under a load, and the car is producing torque. If I cruise its fine, and at speeds under 60mph its fine. So i took those springs out and had them checked by Tein. They were fine. Subaru even installed a new rear diff because they thought that was the problem. Nope, I put the H-techs back in and it still did it.
Now, I sold those exact springs to someone else with the same 04'WRX sedan as me, and they work perfectly. Go figure. So I know that the springs were fine all along and something is wrong with my car in particular. I even went through the hassle to install Eibach pro-kit springs being the drop is less, I figured that the angles on the axels wouldnt be affected as much. Problem is still there. With stock springs my car is fine, but why doesn't it like to be lowered??? Does this look like a drivetrain problem, like bad axels, CV joints, etc, etc...
CAN ANYONE FIGURE OUT MY CAR?????

I have this same problem in my 97 Brighton- GR-2's with stock springs (sagging in the rear a bit) with the exact same noise and vibration you describe. Anybody have any ideas?

I've been doing quite some research about cars in general relating to my problem and I have some ideas/conclusions about it.

I still feel that this is a drivetrain related issue, so i searched about bad CV joints. I found that a bad CV joint usually relates to bad sounds like metal grinding or clunking while turning, etc. meaning it doesnt seems like its my problem.

But then I did some searches on bad driveshafts. I found some similar problems out there. For example, I believe i found something on Jaguars, and people have vibrations at 50-60mph, throttle related, and feel the vibration under the seat. All similar to my symptoms. People have had driveshafts re-balanced or replaced and it seemed to fix their cars. Could I have a bad driveshaft or u-joint?? Please help, and I only prefer responses on the driveshaft idea only, because I think i ruled out everything else such as wheel balancing, alignments, etc.

-WRedX

One more small question besides what I said above, DOES THE DRIVESHAFT ANGLE EVEN GET CHANGED WHEN THE CAR GETS LOWERED? Or do just the axles get changed out of the trans. or rear diff?