Xantia Td auxillary worn bearing noise

[Xaccers]

If it does turn out to be the auto tensioner pulley, then chances are the lower eye of the spring behind the hydraulic pump is worn and therefore the spring will need replacing which adds to the cost. Don't skimp on this though otherwise you'll be replacing the auto tensioner again when it fails prematurely.
Get a cheap 6mm allen key and saw about an inch off the long end, glue it into a 6mm ratchet spanner (and duplicate it to turn the other way), or use tape etc to hold it tight in the spanner. This makes removing the hex bolt that holds the lower end of the spring to the tensioner at whole lot easier (it's still a PITA).
Taping the 6mm spanner to another one to give it a longer shaft may help, as will an extendable magnet, long nose pliers, and a note pad to record all the new and interesting swear words you'll invent.

[bigblackfalco]

Just had a bit of a prod/spray/listen with engineers screwdriver ..... we think its the alternator bearing.

[citronut]

next stage will be to drop the aux belt off and check all the pulleys by hand

[Xantidote]

If it does prove to be an alternator bearing, and the alternator is original and has done 175k, I'd question how much life there is left in the brushes