I just purchased a '95 XJR. There's a rattling sound coming from the front of the engine and some hesitation from the supercharger. Any ideas of what the problem could be? It also seems to smell like gas or some other noxious fume.

How many miles on the XJR? Can you narrow down the area where the noise is coming from, i.e. upper front, lower front, right side front, left side front, etc?

Try using a length of wood, like a broom handle around 12 to 20 inches long and, with the engine running, placing it around on different parts of the engine or supercharger , water pump etc. put your ear on the other end and listen, using the wood as a stethoscope of sorts. be careful that you don't get tangled up in there Maybe this will help you to narrow it down.
Is the noise constant? or only at certain RPM's?

Thanks for the replies. Sounds like the typical upper timing chain tensioner issue. I guess I'll start there.

The idler pullies often make a rattling sound when the bearings wear out. Check the one for the supercharger belt first.

I've had to replace all of mine.

Cheers
DD