This morning I'm leaving my house to go to school. Everything is fine for about 5 minutes, then I started hearing this loud vibrating sound coming from the rear of my car (not in the very back, just in what sounds like the cargo space). I was driving at 55mph in 5th when it started and it happens as long as I am in gear. It goes away when i push in the clutch, as as I accelerate. Once I reach a steady speed, the noise grows louder. I swear I didn't do anything. I was driving very easily, and this just started happening. The only thing I can think of, is when I had traveled about 2 blocks from my house, i heard what sounded like something falling onto the passenger floor. I disreguareded it at the time becuase I just assumed my brother had dropped something, but he said he didn't. Please help me. My car is falling apart more so every day, and I'm out of $$. I may also add, there are no problems with the clutch or shifting.

 

sounds like what was wrong with my car, it was my exhaust.

 

does it get bad when you let off the gas and when you get to higher RPMS in each gear?

 

Yah, but I don't think it's the exhaust. I just had it replaced from the pre-muffler back this summer. What exactly went wrong with yours?

 

Um, it was the 19 yr old exhaust that was on there that had a huge crack down the side of the muffler so when the RPMs were right the two sides of the crack would vibrate together. and there was rust holes everywhere all the way up the exaust system

 

when i got my z it had a bad rear differential it made a similar noise, and only did it while accelerating. You could also have a bad hub, but you would only hear the noise from one side or the other. You can pick up a rear end at a salvage yard for fairly cheap, and I changed mine out myself.

 

I bought my '77 and had a howl all the time and a thump when shifting. The pinion gears were bad so I got a rear end out of a '82 works fine. Check all mounts and u-joints(6 total). I have heard of a rear-end just going bang. If it isn't rear end, u-joints, exhaust then check wheel bearings. Good Luck

 

Put the rear end up on stands, block the fronts first. Start the car and put it in gear lay under the rear and listen for the noise. You can have a friend run it through the RPM's. Make sure the stands and blocks are secure and the car is on a flat surface.

 

The above, U-joints, hubs, exhaust, and the diff itself, are all good things to check. I'll add diff mounts and busings to the list mentioned here already.

The worst case: The last '78 I had did something similar on a trip to Moab once, starting at about Glenwood Springs. I completely lost 5th just short of Moab, and by the time we limped into town I had to stomp the clutch and really jam it into first to get it into the repair lot. The synchro and or bearing from fifth just plain wore out.
I learned several things from this:
1) the original 5 speeds from Datsun aren't very durable. Use a non-Borg, NA 280 ZX 5 speed. Much stronger.
2) FedEx will ship transmissions.
3) Moab is so far from anywhere that FedEx overnight delivery still takes two days.

The long and the short of it: check all the stuff people have listed here, and I really hope it's not your transmission, 'cause that's a pain in the butt you could probably do without in your cash situation.
good luck!
Dave