Noise from the front
There are several possible causes. The connecting rod ball joints might be worn (there are two of them for each rod, they connect the strut to the anti-roll bar). There are two types of rods, adjustable and fixed length; the replacement rods will usually be fixed. If you find an adjustable one on your car, replace both of them.
The steering rod ball joints might be worn.
Check the wishbone rubber mounts. These are nasty because you start to think of them only when they begin to creak, but this is way too late. Long before that they will cause bad wheel geometry and rapid tire wear, not to mmention a strange limited understeering tendency and in later stages (when they do start rattling), rather uncomfortable drive.
The bottom ball joint, although it is quite rare, can be worn, too.
Check the strut cup, the top part that attaches the strut and sphere holder to the body. This is made out of two hemispherical stamped parts joined by vulcanised rubber. The first one to go is usually the one on the passenger side—that's where the outer edge of the road is, together with the majority of potholes. Some very early XMs were known to punch the strut through the hood when this failed, but the construction was changed early on to prevent that. It becomes very noisy on holes and it's easy to diagnose because you can easily move the whole wheel up and down a fraction of an inch and observe the sphere and support doing the same. These will fail in record time if the strut spheres are bad.
It might be the strut itself. In advanced wear (or rust) stages the pushrod has enough wear to rattle inside the remnants of its bearing. Also easily checked by holding onto the strut and trying to shake it vigorously—a clicking sound can be heard. If the car also has the tendency to suddenly drop its front end to the ground for no apparent reason (even if it does climb back up), and the height corrector is OK, replace the strut. Keep in mind that when a strut fails, the LHM will become so contaminated with metal debris that you will be lucky if you only get away with flushing and an LHM change; in most cases, however, you will need to remove and refurbish the accumulator-regulator assembly, the flow distribution valve, and often the brake valve and the security valve as well.
The wear on these parts is excessive if the corresponding sphere becomes flat. Failure of the bottom ball joints or the steering rod joints is dangerous for obvious reasons and are always checked at MOT (or the equivalent in your country), besides, the manufacturers take great care to insure that they don't fail. On the other hand, wishbone silent-blocs, strut supports and connecting rod ball joints are horses of a different color. If the first two are shot, the steering becomes mushy and the car tends to steer itself on bumpy or sloping roads. In addition, this places undue strain on the steering rods and rack, as this becomes the only true rigid element keeping the suspension in its right place. Shot wishbone supports make the situation worse for strut supports and the other way around. Having this checked early on will save you the added (much larger) expense of strut changes and servicing the steering rack.
The strut can suffer due to various reasons: hard suspension due to flat spheres, failing strut or wishbone mounts, old or dirty LHM, or simply due to normal wear. Any problem, as usual, aggravates the normal wear several times.
Unlike the BX struts, XM ones are not (easily) renewable. They usually have to be replaced, and a flush of the hydraulic system has to be done at the very least. A defective strut is usually diagnosed by it leaking into the return excessively, as well as having free-play when agitated (a click can be heard). Defective struts must be replaced as soon as possible because the debris resulting from the wear-out wreaks havoc with the rest of the hydraulics.
Since a certain amount of water and debris, as well as air bubbles in the LHM are normal due to use and wear of the system, a simple operation goes a long way in preventing problems: traveling the suspension fully up, fully down, fully up, and back to normal height again regulairly, say, once in a week or two. Also, the LHM filters should be cleaned regulairly at least, if you are short of cash or for any other reason prevented from doing a flush and LHM change at the prescribed intervals.
The strut mounts can also cause problems on very early Series 1 XMs. If cracks develop in the vulcanised rubber part connecting the inner and outer parts of the strut, they can separate completely. As a result, the strut will be driven through the hood, puncturing or indenting it rather nastily. This setup was modified very early; later variants, although the rubber can fail, will not let the strut pop out of the hood; however, the steering will become very imprecise and noisy. If you drive the car in this condition for a longer period, this will damage the struts themselves, as well as the steering rack, rod and valve.
The danger is somewhat more imminent on the passenger side as that is closer to the edge of the road where most of the potholes are.
Trailing arm bearings
All Citroëns and Peugeots with trailing arm suspension suffer from these problems, although it appears more often on Citroëns, probably due to deflated rear spheres increasing wear on the bearing immensely. The basic problem is that the bearings are not greased adequately. Some people modify the trailing arm by adding a grease nipple (many CX owners do this). However, the first and foremost precaution is having a properly operating rear suspension.
The XM's arms are much more susceptible to extreme wear problems (such as bearing outer rings being completely worn through) because they are made out of aluminum, in contrast to the cast iron arms of the BX. There is a bearing replacement kit but you won't need it generally: the bolt hardly ever gets worn completely, and the bearings and seals are industry standard and readily available from the original manufacturer (bearings are usually made by Timken). The arms will start creaking and getting stuck well before the needles actually get crushed. If you can hear strange noises from the rear end as it rises or lowers, or the rear end remains hard in a bounce test even with brand new spheres, check the bearings. Extreme cases are very easy to spot because the top end of the rear wheels slopes inwards (just put the suspension in high, go back some twenty meters behind the car and observe the wheels).