What's this noise likely to be?

[Mandrake]

Very likely a slipping AUX belt which will only squeal when the hydraulic pump puts a load on it...(it needs 5 HP to drive the pump under full load)

Check the belt condition and tension.

[Old-Guy]

Yup. Auxiliary drive belt slipping. A slipping belt wears faster and faster. It snowballs: worn = slacker = wears faster.....

It needs replacing: if you tighten the worn belt until it doesn't slip, you risk over-tightening and then its the alternator or HP pump bearings that go.

[Xantidote]

Early TDs (such as my 1995 S1) had manual tensioner, whereas later TDs had auto-adjusting set-up. There's another thread at the moment on the auxillary belt topic.

Before you remove the belt, make a mental note/sketch, or even get the digital camera out, to ensure it all goes back the same way. I once tried a new route for the belt and not surprisingly, I couldn't make it fit

[Old-Guy]

Think its manual tensioner, Citroën part 575138, I've checked, £90, ouch! How much tension should there be, and where on the belt?

That's Citroen's price. GSF are good for Xantia parts - a lot less than half the stealer total price for head gasket, bolts, manifold gasket, cam belt etc etc.

IIRC, last I time I changed the tensioner (nearly 6 years ago) I got the auxiliary belt and tensioner from one of the local motor factors. Let your fingers do the walking...

Instructions: there is/was a link to an electronic copy of the Haynes manual on the forum somewhere. Access is from under O/S/F wheelarch (remove lower inner part of liner) and it's pretty simple. This Diagram for belt is for A/C set up; non-A/C the tensioner is in the same place but the belt misses out the A/C compressor.

[Xantidote]

For the manual tensioner, where you haven't got the fancy tool, Haynes gives 5mm deflection at mid-point of longest belt run between, which I take as the bottom length of belt between hydraulics pump & crankshaft pulley - I use a firm thumb pressure to deflect the belt. If too slack, belt will scream under load, but avoid overtightening or you'll put excess load on alternator bearings etc. A bit trial & error, but I've found it works