Land Rover Repair: 1997 Land Rover Discovery Clunking Sound, synthetic gear oil, land rover discovery

Question
Just purchased a 1997 Discovery- Lots to like about it. Would like some help diagnosing the following.

Truck rides great until about 25-30 MPH (although there is an increasing whine from under the front passenger seat that seems unnaturally loud) at that speed, cars starts to buck 2-3 times with a severe "clunking noise." After moving past that speed whine continues but no more bucking...

At 50 MPH it "feels" like the truck should be dropping down into a new gear and sometimes I can coax this by backing of the gas and the RPM's will drop.

I get a sense (not factual) that the noise emanates from the area under the front passenger seat-

When slowing down, I have the same "buck/clunk" as about 30 MPH as I did- perhaps more severely than accelerating-

Don't have a mechanic yet to work on this, willing to work on it myself, just not sure where to head, or what my flow chart for troubleshooting should look like. I am thinking U joints, transfer case, transmission issue-

Ii have experimented-If I glide to a stop and put the truck in neutral the truck does not buck- although the whine is still there. THe transmission in all other ways feels very strong-no slipping

Answer
Hi Don,

I'd take a closer look at the transfer gearbox.  I had a similar problem with my '95 Disco and discovered that I has a slow leak of fluid.  Check gear oil level and try switching to synthetic gear oil to see if it makes a difference.

Best of luck,
JohnMc