Disco 1 Trying to track down a strange issue, help!

I have a 1998 discovery 300 tdi manual. 108k on the clock.

At 2500rpm doing 60mph ish in 5th gear, there is a rhythmic drone/vibration through the cab when the engine is warm.
When it does this, it seem reluctant to push past 60, even with the pump tuned slightly. (it did this before tuning)
If I drop it down to 4th gear, but maintain the same road speed, the vibration/drone disappears which leads me to believe it is engine related.
Compared to my last 300 tdi, with higher mileage, same tyre size, (un modified etc) and the same pump settings, its very sluggish and a bit more thirsty. It smokes slightly more but not excessively.
I have drained the sedimentor of water, run some diesel cleaner through and fitted a new lift pump, fuel filter checked the intercooler pipes, removed the egr valve and relocated the boost signal pipe for the turbo to the manifold. This gave it slightly more poke, but the vibration/drone is the same. I have noted a weep from the inlet/exhaust gasket, and it still has a cat.
The vibration/drone is there very faintly on over run down a hill at the same speed in 5th, but very predominant when under load.
If you get to 3000rpm it almost disappears, and isn't present under about 2500rpm in 5th.
There is no overheating issues, it is not producing blue smoke, using oil excessively or water. Idle is slightly rough but not excessive, but I have raised it slightly to smooth out what was a fairly rough idle.

Hope that's a good enough description. Its literally a rhythmic on off on off timed drone/vibration.

Any ideas chaps?

I would have thought that if it were engine related it would do it in every gear not at specific speeds. Starting with basics have you checked wheel bearings and uj's on the propshafts

I have just had the wheels balanced and the front wheel bearings done.
The uj's feel ok but Im sure someone said on here they can feel fine but fall to bits when off the car! Maybe worth a look, it does feel more towards the front of the car.
I had a dark thought that it may be a clutch issue or worn crank bearings, but it does not judder or slip or vibrate at any other speeds or loads, and the oil light goes out almost before the engine fires which says to me that oil pressure is good therefore the crank bearings or big ends might not be the issue.

it is odd that it only does it in 5th.

Donut ?

No thanks I'm trying to loose some weight.

The rubber does feel a bit soft but it looks in good nick to me.

If it was that or the front prop for definite, I would be half tempted just to by a new front prop and doughnut and be done with it.

Mudron said:

No thanks I'm trying to loose some weight.

The rubber does feel a bit soft but it looks in good nick to me.

If it was that or the front prop for definite, I would be half tempted just to by a new front prop and doughnut and be done with it.

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Take the rear prop off and drive around if the noise has gone you've only got half a problem ...

So looks like in looking in the direction of props and joints. Thanks for the advice, il take a closer look

I'm thinking gearbox, not props. It is reading as a gear-dependent noise, not road-speed or engine speed dependent one.

However... in the interests of belt and braces, check what RPMs give you the noise in 5th, then drive constantly in other gears at that engine speed, after all we do tend to just rev up through the gears rather than keep the engine speed constant.

That the noise disappears when in 4th at the same road speed rules out props, diffs, bearings etc to me as they are all roadspeed dependent.

I can now confirm that the same rpm in other gears does not produce the drone. Its is only in 5th at 60 mph at about 2500rpm.
The higher the load, the loader the drone, I had a trailer on the back, maybe 400kgs total weight and load, and it made the drone slightly louder. The rhythmic part of the drone is constant and consistent in timing at that speed and rpm. On off on off on off etc at 1 second intervals.

Whatever it is, it seems to hold the old girl back.

Would a restricted fuel line cause this do you all think, I drained a whole sedimentors worth of water out of it, but have not split it to clean it. Fuel from the tank flows into the sedimentor easily enough because after the water ran out the diesel flowed.

Was there any water in the bottom of the fuel filter then ?

the old fuel filter had a little bit in it, maybe a 10ml tops, but the sedimentor was pretty much full to the brim as a whole cup full of water came out.

Well, it did it's job ...

it did indeed

Because its a rhythmic resonance and because it still has the cat, I am going to fit a non cat front pipe to see if its that. sometimes the flexi can seize and cause a drone at certain speeds etc so may be that. Can only do good even if it doesn't solve the issue. I'm going up to the LRO show next week so I don't want 4 hours of that noise!!!

Ok so Iv cleaned the sedimentor out, confirmed fuel supply/air in fuel or air supply isnt the issue.
Changed the inlet manifold/exhaust gasket and fitted a decat pipe. Its made no difference to vibration although it pulls s bit smoother.
It almost feels like a missfire, im now wondering what the spray pattern and pop pressure of the injectors look like. Would a leakoff flow test give me much info?

Mudron said:

Its is only in 5th at 60 mph at about 2500rpm.

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I can confirm that my '97 Tdi does the same, a rhythmic droning at those revs at that road speed only in 5th gear, I have owned the vehicle 5 years and it made the noise from the time I drove it home, I really think it is either a diff bearing or the gearbox bearings involved in 5th gear, my Disco just keeps on going, one day it may go "bang" then we will know for sure what it was causing the trouble, but I've always believed , if it ain't broke don't fix it".
Apart from the annoyance of the noise at just under to just above the 100Klm/hr mark, it is a minor inconvenience.

Glad im not the only one, I had previously linked the vibration/drone to lack of power. This is not the case as it now pulls like a train since the decat was fitted (the cat was the original LR one and was almost complete blocked)

I am hoping that its not gearbox or transfer box related, it does not make the noise in 5th in low range. It is definitely forward of the driver ie/engine/front diff/drive shaft etc, passengers in the rear have commented its coming from the front.

I might try a molyslip gearbox additive to see if it lessens the drone.

Right, update, I have ruled out anything drive train related as the engine appears to be slowly getting rougher.
It has plenty of power and doesn't smoke at idle or does not smoke much at full throttle.
At idle, as the revs rise when you press the accelerator, you an hear the engine slowly get rougher in vibration up until about 2500rpm where the roughness harsh vibration peaks, it then progressively gets smoother again as it goes higher, 3k 4k etc.

This coincides with driving, as you hit the 2500rpm mark, the engine has a lot of vibration, so much so that it rattles the dash and drones through the whole body, the engine note changes too.

It still pulls cleanly through the revs, but cruising at 60mph is like being on a speed boat with the vibration and noise, its driving me nuts!

Does anyone have any suggestions as to what it might be??? I cant understand why that particular part of the rev range is so significant???

You may find its a crappy injector and the engine at that speed is not balanced , so 500 rpm up or down its ok ,, i have had many diesels that don't like a certain rpm ,,We have a RV that hates 2200 rpm ,,, its slow to get to 2500 and drinks diesel like a drunk hooker ,,,
But at 2700 its smooth ..Maybe its just the system ,, or even the injector pump timing ,, i advanced mine a few deg and helped a lot .