That will almost certainly be the rear axle carrier bushes. Upgrade to polyurethane and they are stronger, last longer and easier to fit.

POWERFLEX BMW E53 X5 Rear Subframe Bushes, Tool avail. on eBay (end time 10-Nov-09 12:34:50 GMT)

X5Girl, xxx

__________________
Black E53 V8 on 20's, lowered, drilled discs and poly bushes, strut brace and adjustable sway bars.