MYSTERY ISSUE. Doing my head in. Please Help!

[PMC]

Ok,

Driving me absolutely mental, to the point where I'm ready to sell it (MOF has said I'm not allowed to sell this one) but I don't want to be spending money replacing things one by one until this issue is sorted. Haven't been able to sleep, just keep trying to think what it could be and pinpoint where this issue is.

It is a 98 GU 4.5 Petrol, manual.

Nothing has been changed prior to this starting.

So it started almost 2 weeks ago, and has only gotten slightly worse.
When driving, Troll is fine until over 50kph. If feathering the throttle, very lightly on and off, there's a grinding noise/slight vibration (best description would be more of a growling) that can be heard/felt through the vehicle. Under acceleration I can't detect it, and off throttle (engine braking) I can't detect it either.
Also, once over 60kph or so, if the clutch pedal is held in, or if gear lever is put in neutral with clutch pedal out, noise is constant.
It is more prominent at higher speeds.
I can't pinpoint the origin, other than I am pretty sure it is in driveline.

Have been to 3 different places, all have test driven and inspected. Here's their provided info.

1. One says that the front shaft can't cope with lift, needs a second uni joint on one end (double cardan???)

2. One said clutch bearing on its way (they also said uni joints and all wheel bearings are fine)

3. One said rear diff needs rebuild

Don't know which one, if any, to go with.

I have a bit of mechanical knowledge, don't know all, nor have very in depth knowledge, but have thought about a few things. Pretty straight forward I think (in my mind it all makes sense).

Please correct me if I'm wrong anywhere.

With 1. Troll has been driving fine with lift, etc, nothing had changed just before the noise started, plus transfer in 2H with hubs on Auto, would mean front shaft isn't moving.

With 2. 'growl' is worse at higher speed, where as the clutch bearing would only turn as fast as motor rpm (which is lower in a higher gear at speed) plus the 'growl' doesn't change with engine speed.

With 3. I am leaning more to this as the explanation for this 'growl', mainly due to the speed factor, being that rear tailshaft, rear diff, rear wheels (bearings) are all increasing rpm with more speed.

The thoughts I have on 3. also made me wonder if it might be front wheels (bearings) as these also increase rpm with speed increase.
HOWEVER
With front wheel bearings, would the feathering of the throttle on and off (on and off load) cause front wheel bearing issues to become apparent as there is no drive on these wheels?

Any info, suggestions, help in general would be appreciated, preferably before I go looking for a box of matches.

If I have missed anything, or any additional info is needed, please tell me, and I'll do my best.

Thanks in advance.

Cheers,

Evo

G'day Evo,

If you can, choc your front wheels, jack up the rear diff so the tyres are a good inch from the ground and support with stands, leaving the jack with most of the load on it.

Start Patrol and use first to second to third and then to forth gear gentle pushing the speed up to the 6o km mark that you have been hearing the noise. Feathering the throttle, very lightly on and off to see if it makes a noise!

If it does, we know the noise is from the motor back to the rear end!

Put Patrol in neutral and apply your brakes gently to stop rear wheels from rotating, note if any noise occurs whilst braking. If noise occurs check both calipers,disks and wheel bearings.

If there is no noise, slowly get out of the Patrol whilst engine is idling and grab the driver’s side rear wheel and hear for any metallic grinding noises, do the same for the passenger side rear.

What you are doing is placing a shock load to the diff and rear wheel bearings when you grad one of the rear wheels, if a noise occurs, then we now know the noise is coming from one of these component’s. Its then a process of elimination!

1. Stop Patrol, remove both wheels. Use first to second to third and then to forth gear gentle pushing the speed up to the 60 km mark that you have been hearing the noise. Feathering the throttle, very lightly on and off to see if it makes a noise!

If there is a noise, it’s then coming from either the drive shaft or the diff.

If there is no noise, it’s then one of the rear wheel bearings.

Fit the passenger side rear wheel back on. Start Patrol whilst engine is still idling and grab the passenger’s side rear wheel and hear for any metallic grinding noises.

If there is, then then stop Patrol and refit driver’s side wheel and then remove passenger side rear wheel. Start Patrol whilst engine is still idling and grab the driver’s side rear wheel and hear for any metallic grinding noises. If there is no noise, then it’s the passenger side rear wheel bearings.

IF there is no noise coming after these tests, whilst the rear is still on stands, with another jack and another set of stands jack up the front, so the front wheels are off the ground
.
Start Patrol and engage high-range, gently engage first to second to third and then to forth gear gentle pushing the speed up to the 60 km mark that you have been hearing the noise. Feathering the throttle, very lightly on and off to see if it makes a noise!

If no noise, slowly get out of the Patrol whilst engine is idling and grab the driver’s side front wheel and hear for any metallic grinding noises, do the same for the passenger side front.

Unless you have the proper safety equipment, do not attempt jacking the front wheels. Go to a Mechanic who can perform these tests on a hoist safely.

By doing the test on the front wheels you are trying find out which wheel bearing is at fault, in a workshop environment, I would disconnect the rear tail shaft and then conduct the test, it eliminates the rear wheels being engaged and places greater load to the front diff and front wheel bearings.

Questions:

1. Did you see the mechanic conduct the tests on your Patrol? If so, did he have the vehicle on a hoist conducting simular test’s that I have just mentioned above?

2. What year is your Patrol? I do know that the synchromesh gears did play up with some of the early GU’s 1997 to 2001 4.5 petrol models. Especially third gear synchro.

3. When was the last time you did a water crossing, especially salt water?

PS, If you cannot get a friend who is mechanically to conduct the tests above safely (due to your back), then please go and ask your mechanic to perform them for you on his workshop hoist.

Regards,

RLI