Hi,
I have had a similar problem, been chasing it for years. Have carried spare serpentine belts just incase. Cut to the chase I just had 200,000K service replaced timing belt and it was found that the bearing in the timing cover, (not replaceble) was stuffed. So a new timing cover and its all good.

Cheers
Ralf

Currently have the same noise. Thing is though never did until I put a new belt on. Do some belts have to be worn in a little before the noise is gone. I just changed my tension pulley about 6 months ago and a LR one at that.

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95 Disco 300tdi - Now with MORE BOOST
Cheers Simo

some belts are rougher than others. D300Tdi check to see if the back of your belt is smooth and shinny if it is change it for a rough backed one...worked for me

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Our Land Rover does not leak oil! it just marks its territory.......

The back of the belt is smooth and shiny and i just got a spare one today and its the same as the one i have on. the brand of belt is a Dayco ERR 3287.

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95 Disco 300tdi - Now with MORE BOOST
Cheers Simo

Well paint me pink and call me a @oofter! - it works.
Thanks Geoff
p.s. I cost me 20 cents (dropped the first coin and couldnt find it )
but that's less than 0.5% of the cost of a new belt!

That bearing is replaceable, there are a couple of guides here in the tech section.