For the past six months I've had an odd rattle coming from under the bonnet -driver's side. I would have guessed a suspension component except it seemed to be high and close to the firewall and sounded like someone shaking a plastic container that has an object inside it. Coincidentally I had the same noise for several years with my last XJ8. Today I decided to find the source, and after some detective work I removed the lid/cover over the brake fluid and washer fluid containers, and laying inside that compartment was a large rubber/plastic part I couldn't identify. It wasn't connected to anything and was an obvious source of the rattle (which has disappeared since I reconnected the item). I found a part number on it and searched the internet until I found it. Turns out to be the windscreen water drain hose. I reconnected both ends to their fittings and secured it with the little clips and voila - the noise is gone and the mystery is solved.

 

I'm having a similar noise issue. I'll have to look into this to see if it clears mine up. Thx!