Mike,
An easy way to tell whether the noise is coming from a belt is to wait for the noise to happen again. Once you hear it, while the car is rolling, and if it's safe ofcourse, put the car in neutral and give it gas. The noise should change in character with a change in the engine speed. If not, then it's not from the engine but rather from the driveshaft. I have done that and the noise did not change. That's how I know it's not a belt. Hang in there buddy, we'll work this thing out. You're the biggest advocate to the X-type. Don't get discouraged. We understand your frustration. It's too bad Jaguar won't take lessons from Lexus re-customer service. I'll keep you updated.

Perhaps you should return to your local dealer & have them check the propshaft-some of the early cars have had a problem with hanger bearings-as evidenced by red grease coming out of the bearing-ps the part has been revamped