Hi all! My Sporty has developed a strange problem and I am hoping someone can help!

When driving, once I reach about 45 to 50 MPH, I start to feel a vibration in the drivetrain, and it gets worse if I go faster. It does it when power is being applied to the wheels to move forward, so if I'm cruising or accelerating it vibrates, but if I let go of the gas and allow the car to engine brake, the noise and vibration stop.

Not only that, but if I accelerate to 50 or 60 MPH and then put the car in neutral, the noise and vibration will continue. HOWEVER, as soon as I put it back in gear, so that now I am starting to engine brake, the noise and vibration stop. Then, if I put it in neutral again, the noise will be gone.

So, the noise is tied to what speed I'm going, not the engine RPM. Engine RPM has no bearing on it, just speed. The other factor is whether the drivetrain is loaded up or dragging / engine braking. Noise only happens when the drivetrain is loaded up.

It doesn't matter what gear I'm in, just the speed. I can go the same speed in 5th or 4th gear and the noise is the same, or even if I'm in neutral the noise is the same, but noise goes away if I engine brake then put it in neutral. As soon as I engine brake even a tiny bit, bam the noise is gone and stays gone until I load the drivetrain (accelerate).

Any ideas? My first thought was the rear differential going bad, or maybe out of balance somehow, or maybe the driveshaft out of balance?

Also the transmission, transfer case, and differential all have enough gear oil in them, I checked that. The gear oil in the rear diff looks relatively clean to me, but has a slightly reddish-brown look to it, is that normal?

Any help would be appreciated! I'm about ready to take it to a shop if I can't figure it out very soon.

OK there is more trouble!

I have the 5 speed manual transmission and the gearbox is leaking gear oil out of somewhere up high on the gearbox, seems almost like it's leaking out the TOP. I guess I need to pull off the console and take a look up there. From the pattern of where the gear oil is, it seems like it's leaking from near where the shifter goes into the gearbox. How can that be?

The gear oil was really low too because it had been leaking for a few days. I filled it back up for now but I'm trying to find exactly where it's leaking.

The weird thing is I thought it was leaking from the sensor that screws into the passenger side of the gearbox, because that thing had worked its way loose so that it was only hand-tight. I cranked it on tight so it's not leaking there, but the leak didn't go away. I'm not sure if it was ever leaking from there or if the leak was always coming from on top of the transmission.

Update: OK this is weird!

Driving on the freeway, the vibration was getting worse up to about 60 MPH, then stayed the same until about 70, then all of a sudden, while I was cruising at about 70, there was sort of a little clunk and the noise stopped!

After that, the noise stayed gone for 5 minutes or so, while I cruised between 70 and 75 MPH. I tried accelerating, engine braking, etc. and the noise was staying gone. However, the gearshifter was shaking back and forth more than usual, sort of wobbling around rythmically.

Then after a few minutes, the noise came back <img src="/forums/images/graemlins/frown.gif" alt="" />

Doesn't this make it sound like there is a problem with the gearbox? Maybe some bad bearing somewhere that sometimes pops in and out of place or something?

Two things I'd check

1) the driveshaft/u-joints, new u-joints and balancing should run about $150-175

2) the transmission - likely primary shaft wobbling in case. This could run $500-1500...

Thanks DRX350, what I can't figure out though is what the gear oil leak has to do with any of this. Would the vibrations somehow cause the gear oil to leak out the top of the gearbox or something? Or do you think they might be 2 separate issues?

+1 on the drive shaft u-joints. I had a nasty vibration acting similar to what you are describing and it was a u-joint. Block the wheels, put the T-case in neutral, get under it and play with the drive shafts looking for slop in the u-joints. Important that your t-case is in neutral so the shafts can move freely.

Can probably pick up a u-joint from a parts store for under $20. Wouldn't worry about the balancing unless you replace it and it still vibrates.

The leak is probably not related. I have the same leak with my 5-speed and no issues caused from it that I am aware of. Have not looked into where it's coming from, so if you figure it out anytime soon, post it up.

+2 on the u-joints. Last summer I was chasing my tail on a vibrtion. Swore it was comming from the front end. My front wheel bearings were loose and bad but that didn't fix it.

I then crawled under and checked the u-joints and man did the rear drive shaft have some play. One of the u-joints had no needle bearings left in it. All it had was metal dust <img src="/forums/images/graemlins/zombie.gif" alt="" />

As for the leak on top I believe there is a gasket where the shift lever bolts to the transmission. You'll need to pull the center console out to access the shifter.

^^^THIS

Unfortunately I don't have time to work on the car for the next couple of weeks and I need it fixed asap. I gave up and took it to a shop. Hope it's nothing serious. Right now at work we have urgent stuff going on and it could potentially cost me my job if I have to spend a lot of time playing mechanic <img src="/forums/images/graemlins/frown.gif" alt="" /> Normally I'd do my best to fix it myself but I just can't deal with it this time. I'll post what they find.

Bad news. The problem was a bad u joint, but I didn't fix it soon enough and the vibration actually cracked the transmission case, they showed me the crack. That's why the gear oil leaks out.

Kicking myself right now for not noticing and fixing the vibration sooner, and looking into the option of putting in a used/pulled transmission. The shop of course wants a lot of money, what they would recommend is rebuilding the whole transmission in a new case, they want about 2200 for the whole job if we went that route. Not gonna do that. If they quote me a decent price to put in a used one, I'll go that route, otherwise I'll tow it home and park it until fall when maybe I'd have time to do the swap myself and the weather will be cooler.

I thought the vibration was the engine running rough because I had a check engine light and ended up finding a vacuum leak, but it still was vibrating, and I thought maybe I had a bad injector, or spark plug or something <img src="/forums/images/graemlins/frown.gif" alt="" />