Does this sound like a driveshaft out of balance or tranny?
 

Picked up a GSL-SE. Been driving it. Runs alright, but there's vibration when in gear, that increases in severity during acceleration. Frequency grows with RPMs, and it's pretty high to start with. I feel it through the steering wheel, but most of it is apparent through the shifter, which rocks and vibrates pretty brutally.

Vibration is reduced when there's no load on the engine. If I'm in gear and rolling downhill with no throttle it's a lot more subtle. I haven't looked under the car yet, but I will tomorrow. Just not sure what to check for. Could it be that the tranny is out of balance or damaged? Because the vibration frequency is a bit too high to be the driveshaft.

Motor revs smooth in neutral, btw.

It could be the Pilot bearing, this is the bearing that the transmissions input shaft rides on to keep it in the center of the E-Shaft.

If this bearing goes out it can cause major vibration higher in the RPM range.

Does this vibration happen as soon as you start rolling, if it is does it feel like it is in the front or rear end, possibility it is a bearing in the rear end, I have had that happen to me on a 1st gen.

Hope this helps, and welcome

It happens when I accelerate assertively or on the gas in high RPMs. Feels like it's in the front end. Transmission/engine area.

The driveshaft should be easy enough to pull and check.

Ok, so I don't think it's the driveshaft. Vibration frequency is a lot higher than rotation frequency of the driveshaft. Plus it's RPM based, and not speed based. I don't think it's the motor, either. Revs smooth in neutral. Unless there's something wonky under load.

So that leaves the clutch and the tranny. Previous owner told me the clutch has been replaced not too long ago. It could have gone bad, yes, but I doubt it. Tranny is the only thing left. Now, what part of the tranny could cause such vibration?

Also, what does a bad pilot bearing feel or sound like when driving?

Bad pilot bearings only make noise when the clutch is pushed in, RPM based could still be the driveshaft because it also depends on load of the car. If you put your car in neutral and there is no vibration but there is vibration while driving that points to the driveshaft.

does it do it at highway speeds out of gear? your tires could be out of balance :)

Tough to say what it could be without hearing it. Did you say it happens if you are parked and revving? I know you said RPM related, but in what gears while accelerating? 4th gear is the only direct gear.. it is 1:1, and will display different noise characteristcs than other gears. What clutch and driveshaft are on the car? Too light of a clutch/flywheel and too light of a driveshaft will cause all sorts of noise... As dumb as this sounds, make sure the tranny has oil in it, and while you are at it, thicken up the oil.

-Bart

There's nothing to hear, actually. It's a silent vibration. The only sounds that exist are the sounds of rattling trim, radio buttons, and gear stick. If I lay on the gas into higher RPMs, there is an extra rattle, but it sounds more like plastic or something. I'm guessing parts of the body around the engine bay vibrating.

It's not the tires, because it doesn't happen in neutral, and it's less noticeable when I'm simply off the gas in gear.

And yes, 4th gear is the most prominent, since everything is in sync with the motor.

Good point about tranny fluid. Will check. Thanks.