Drivetrain Humming / Vibration coasting in 2WD but not 4WD

[blackmo92]

Hi All,

I've been experiencing some humming / vibration when coasting in gear.

Most noticeable in 3rd & 4th <2000rpm, dissapears once I give it some gas or push down the clutch.

Interestingly this vibration / noise is completely nonexistent in 4WD. All fluids were changed about 3 months ago, this vibration has existed since I had the car, never thought too much of it until I drove a different manual car (this is my first) and realized you should be able to coast in gear basically until the car stops moving (obviously it would stall).

Appreciate any suggestions!

[Rigger]

First bet would be to check the universals. Give it a good grease if they have nipples.

[Mud Dog]

And check the centre carrier bearing on the rear drive-shaft for excessive play. It has a rubber carrier ring that allows a little play, but no more than 5mm each way would still be acceptable and even then it should feel like fairly firm movement (not sloppy). Also check the splined yoke / slip-joint for wear (side-way play) - greasing it helps as well (there is a grease nipple for that as well).

[blackmo92]

Apologies for the slow response, I got under the car and inspected the uni's (all seemed fine!) however I was unable to great the rear uni joint, it just wouldn't accept any grease. I sprayed it with some WD40 and poked a small screwdriver though the nipple to see if it was blocked which it wasn't. In the end I got frustrated with all the grease getting on me and gave up

There does seem to be a bit of play on the rear tailshaft, but its probably less than 5mm? I have tried to film with one hand (poorly) but you can see the play at about the :20 second mark.
https://www.youtube.com/watch?v=w0-KaHB8OvA

What exactly occurs when switching to 4H vs. 2WD, does the front diff / driveshaft only spin when 4WD is engaged?

I've looked at the effort involved in replacing uni's and it doesn't seem too difficult, in the meantime would there be any issue driving in 4H with front hubs unlocked other than extra fuel consumption?

[Mud Dog]

Can't really see if there is play on that video clip. The play that you felt, is it on the centre carrier bearing or on the slip-joint?

You can drive in 4WD with the front hubs unlocked without causing any damage, but I wouldn't do that for too long a period - just until you can get to the bottom of the problem should be OK.