I have an '04 XJR w/ 50,000 miles on it. Recently, I've noticed a definitive low pitched rattle @ very specific RPM's in all gears. It's loud enough to hear with all windows up and the radio off. When accelerating through 2000-2100 RPM, this rattle noise occurs but I cannot pin point what's causing it. It seems like it's coming from the rear passenger side, but the more and more it occurs, I want to say it's point of origin is near the dash. When hitting the gas in neutral, I've yet to hear to hear the noise which makes me even more confused. Help!!!