So far what I know is: I only noticed it once up to 199+ degrees. It obviously comes from under the truck, it's hard to hear when on top of the engine. When shifting gears, it either goes away for a moment, like from park to reverse it completely disappears for 2-3 seconds before coming back. Same with the other gears, but not as distinct. I'll try to be more precise with temperature in the morning. I managed a video.
Any thoughts would be appreciated.
Edit: 2004 disco 2 with a '99 p38 4.0 engine. Roughly 212kkm.
Also sorry for the spoiler, on mobile and can't seem to delete it.

 

You need to keep the temperature below 190 degrees, you need an inline thermostat with a 180 degree thermostat.

 

Does it disappear in neutral ? Could be an iffy flex plate

 

Not really. I ran it today, let it warm to 194 (as hot as it would get) and nothing. Ran it at ~1500rpm for 1 minute and there it was. On my way home nothing. And it got to 199 give or take. I ordered the inline anyway. It's been on my list of things to do, so might as well. Also found an ACE leak is occuring and needs to be sorted... Yay!