I've got a thrust bearing noise when taking my foot off the clutch at idle which by all accounts is not unusual, what does concern me however is a similar sort of noise under load in third and fourth gears.

Is this likely to be the thrust bearing or something more sinister?

could be i hope so for your sake mate, have u dropped the g box oil recently

I've only done about 50 miles in it since I bought it, doing all transmission oils on Saturday morning.

check the t box and g box fluid out when u drop them it may tell a story the noise may go with new fluid but i doubt it mine has made the same noises for a year now and the clutch is so hard compared to my other truck something is not right but it goes

I'll see what kind of crap comes out in the oil, fingers crossed!

I guess it could also be something on the prop?

or both! i know i need a new center bearing on prop that is another noise

My old truck made a strange noise that got worse, turned out to be the clips for the return bearing. Could be them

Not likely to be this but my input bearing collapsed on my V5M21 gearbox. made a funny noise in 3rd and 5th. You should have the stronger V5MT1 box if it is a classic though. Could also be the spigot bearing if somebody changed the clutch and didn't bother their arse pulling the old one out.