Two things occur to me.

1. not likely to be strut mounts as they fail catastrophically rather than rattling! Run a broomstick across the car from mudguard to mudguard and check that there is clearance (1-2 cm or a bit more if I recall) between it and the nuts at the top and that the clearance is the same on both sides. Be very scared if one of them is pushed up!

2. balljoints can give intermittent "cracking" noises if the lubricant has dried out. I've drilled and fitted grease nipples before now to fix this one (not on a Xantia though).

Good Luck.