Hi Ken,

Unfortunately I never got to the bottom of the problem. My foot now just naturally avoids the trouble rev range/load combination. 

Hi Dan,
I've been in Bristol today with with middle son for an operation on his hip. So when the wife suggested I go and annoy someone else, somewhere else, I went to see Cam Tech Craig (a good bloke who made me feel very welcome & who makes a good coffee - thank you Craig ). Craig drove my truck and his opinion matches my worries.....transfer box. He suggested I went to Bristol Transmissions, which I did, & they are of the same opinion. The noise has become far more apparent today since I have just changed the front & rear diff oil and transfer box oil which, instead of improving things, has exacerbated the problem. It is now very definitely in the centre of the car & low down, and does not sound like the diffs. I am really hoping it will survive the nearly 200 mile trip home!??!
You are on similar mileage to me (>186K miles)so your problem might well be similar.
Cheers, Ken
(P.S. thanks again Craig. ) 55 S/C L322 Zambesi Silver.....GONE!!
56 S/C L322 Java Black......surviving at present...just!
04 N/A 4.2 V8 S Type Jag
1961 reg 3.5 V8 Bowler Tomcat.....Sold
Numerous Classic (rusty!?!) Minis