Weird noise-need some ideas

[madkaw]

Some of you have been probably following my zcon thread, but I have been doing a complete resto and I am nwo just driving the car around for the first time in years. I am very happy with the way the car handles and performs, but I have one of the 'weird noises' that I am trying to figure out.

The noise seems to be coming from the rear and it sounds like a tire with a slight imperfection or slightly out of balance-but there is no shimmy or any vibration. The noise comes in around 55 and goes away around 70mph. Here is the weird part for me; at a constant speed of 60mph where the noise is the greatest in 4th gear, if I shift up to 5th gear the noise almost goes away. Shift back to 4th and the noise is right there again.

At first I was sure it was a tire balance issue and had the NEW tires rebalanced and I even rotated front to rear-no change. I then suspected maybe that a halfshaft is out of balance, but the discovery of the gear change - changing the noise- makes we question all of my assumptions. Or even if the noise is coming from the rear, maybe it just sounds that way.

any ideas??????

[d240zx2]

Have you considered exhaust resonance or a loose exhaust hanger? Since it goes away in 5th at the same speed, the only thing that's changed is engine RPM. You may want to check the noise that way again, but in third gear, then to 4th and on to 5th. See if the noise comes back....

[madkaw]

The noise seems rotational-like as in a tire balance issue-so i can't see how it could be exhaust. The only resonance issue I have is at 2500-and it's just a low rumble.

I also thought it could be a rear bearing-but once again the sound is effected by gear selection.

Could the gear selection cause a slightly different load on the driveline which would make some noise more apparent?

Edited July 14, 2010 by madkaw

[d240zx2]

Given a constant speed, the torque required to maintain that speed from the driveshaft rearward should be constant.

[madkaw]

Exactly! That's why I'm baffled. But is the torque the same given the different rpm at the constant speed. The engine would be putting out more torque at 3000rpm then at 2400rpm-4th gear to 5th respectively. So I maybe going off the deep end here, but does the difference throw the shafts(driveshafts) into an angle that maybe promotes a slight out of balance-thus the noise.

A liitle background here

2.4 motor

81 zxt bwt-5 speed

driveshaft shortened and balanced

halfshafts had universal joints replaced around 5k ago-but don't know if they were balanced-doubt it.

New tires, wheels, wildwood brakes

[Shade133]

look at it this way.

your engine is moving at 3k RPM. when it goes through your transmission the rotation of the drive shaft coming out of your tranny is obviously not 3000 RPM. and once it goes through your differential your 2 rear drive shafts will turn at the same speed as the tires.

so if it only changes with RPM. (and load seems the un likely issue here since the noise is a somewhat noise like you have a buble or wabble tire/wheel out of ballance? if im picture the noise its like a "WOM WOM. WOM. WOM. WOM WOM WOM" changing from fast to slow depending on speed right? well since your going the same speed but only changing RPM's

that leaves the tranny and the rear drive shaft.

rear tranny bearing maybe is dry and is litterally about to go out. or maybe your drive shaft being shortened and balance needs to be balanced WITH the u joints on (which is how i thought they always did it)

its definately not an issue with tire or rear end or the 2 smal drive shafts if it changes with RPM.

so your right thinking it might be the rear drive shaft going to the diff from the tranny. but it could also be the tranny to.

or maybe its just how the ZX tranny's sound .

i cant give any further info without knowing the condition of the tranny, as if you fully rebuilt it before putting it in or took it from a wrecked car that had good miles or what.

[madkaw]

Tranny came from a cherry zxt that was wrecked. It has a total of 60k miles.

If I get under there and start trying to move things in the drivetrain-it seems to have some slop-but anything I would worry about. Maybe I need to check out the pinion area for slop.

[Shade133]

its possible.

if all else fails take it to the driveline shop you got the drive line balanced and have them check it again

[madkaw]

I might have found the culprit. I have some definite play in the diff side bearings. The left side being worse. The main driveshaft is solid with no play and balance weights are in place.

I imagine there should be no-up/down play in the side bearings. This is a lsd with unknown history. I guess I need to see what is involved in changing out the bearings. Changing the load by shifting is maybe putting more or less load on these bearings-thus the noise-what do you think?

[Shade133]

that COULD be possible, i can see that happening if the bearings are bad enough. didnt think of that one.

good catch. id get the diff checked out. maybe rebuilt