I had a similar issue on my Doblo 1.9jtd.

Turns out the clutch & DMF were in need of replacement.

After that, the harmonic hum disappeared.

It was more of a thrum than a hum, but worse when under load (caravan on the back).
 
Well I've done a leak off test and it does appear there is a vast difference between each injector. and the pump timing is ever so slightly out. That will fix the engine vibration when I get around to it.

Now I have what appears to be a 70mph wheel wobble that comes and goes. So I am now suspecting swivel bearings. The trip to the LRO show this weekend made traveling at 70mph interesting, one min the wobble is there, the next min it comes in with a vengeance!
 
Are you sure its not a simple as a bad shock absorber ,,I have had one that the wheel would bounce violently at certain speeds if the road has small corrugations , But the rest of the time was ok ??
if you know some one with a gopro camera it may solve the problem of what is making the shake
 
Something else to look at for sure. That's a really good idea with the gopro. Is weird how it suddenly comes from out of no where when you are cruising at or above 60, like a wave going through the car, and then goes again and wont come back for ages.
Shocks are the originals from the factory, and it has done 108k so it possible!
 
I also seem to have a very similar drone over 55 mph, but in an auto.

I bought mine a couple of months ago but didn't notice anything on a test drive. It also sounds a bit harsh but as yet I can't decide what might be wrong. I don't think it's engine related and there are recent invoices for front wheel bearings.

I haven't looked closely at my Disco yet but we have an old one on the farm.....is the transfer box difficult to remove as I could exchange a few items?
 
Transfer box like everything a disco is just nuts and bolts, fairly simole to remove but its a big old lump that will require more than one person or a jack and make shift cradle
 

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