I have a 73 240Z with a ZX five speed & 3.54:1 diff. Lately when I shift into fifth I hear a noise coming from sort of behind me....like it's the rear end or an axle, but kind of on the driver's side and not as far forward as the gearbox. However, this makes no sense because it's definitely drive-train noise and if it's the rear end, wouldn't it make sounds in all gears, not just fifth? It's not quite a grinding (like bearing failure) but more of a rumbling type of sound. I know sounds travel and can be hard to pinpoint with all the other noises happening while driving but it's weird that it's so magnified only when in 5th gear. I'm reluctantly thinking it's the trans....but, Any other suggestions?

Does the noise occur immediately upon shifting into 5th or after shifting into 5th and releasing the clutch?

It occurs after shifting & releasing the clutch. 1-2-3-4 no noise...5th: noise. ?????

Once the drivetrain is under load, it starts rumbling & grumbling. But only in 5th.

But not just lugging the engine due to upshifting too soon and dropping the RPMs too much? Does the noise ever go away at higher speeds?

Nope, stays with me in 5th. I haven't lugged the motor in that gear yet, so I might try it and see what happens...

Does it still make the noise when you put it in neutral and release the clutch at the same speed (70 mph or whatever speed you're going)? What you're describing sounds more like a 5th gear problem than anything else.

If you maintain the speed at which you hear the noise, and downshift to 4th, does the symptom go away? If the symptom persists, check your driveline and make sure it's tight. Inspect the condition of the u-joints.