Being a new owner of an older machine, I'm not sure what is normal and what is not.
Yesterday, while driving about town, I thought the engine (4.0L) began to sound 'different'. With windows up, stereo off, and gentle running, I thought the engine sounded just a tad off...almost as if it wanted to start to miss on a cylinder. There are no CEL or warnings of any kind, but as I listened for a while, I got the name for the sound....It almost sounds as though it is a sewing machine. Very smooth, yet this slight sound as though it could be missfiring. There seems to be plenty of power, and for all I know, this is a normal sound, yet my gut says to be aware of it...
Any thoughts on this?

 

The coil on a spark plug failing is a fairly common reason for misfiring

 

For some reason these cars take a while to detect a misfire and turn on the check engine light. I had a coil causing a misfire that didn't set a code, until suddenly it said all cylinders were misfire although it was just one.

 

OOOH!
I am not usually a worrywart about these things, BUT I am afraid you MIGHT be hearing the sound of a secondary tensioner beginning to fail.

If you are not sure they have been replaced, check the archives, especially those of folks who have had a failed tensioner, then figure out if yours have been replaced. If not- fair warning!

 

Speaking from experience and also being a new owner of a early model V-8, it definitely sounds like the tensioner. I had to replace mine after only driving the car about two miles after purchase. $2600 repair replacing all four with the new and improved tensioners. Sorry for the bad news.

 

If you let it go and have a valve crash, it is WAY WORSE!

 

Then you can do the Chevy engine swap! Being a 98, they are right about finding out if the secondary POS tensioners have been replaced. I did my own in like 2-3 hours for about $300+ in parts. The price of VC gaskets alone blows my mind on these cars. I opted to not put in the VCT solenoid seals and now look at me... they are leaking! So now I get to pull the covers again sometime to reseal everything again, grommets too, which I also skipped out on. Good luck!

 

I sure wouldn't have bought a Jag if I wanted a Chevy engine! I already have one Chevy engine, and that is enough.
The entire timing chain system was rebuilt about 20k ago by the Jag dealer in Nashville. Unless he put the same exact system back in without upgrading it, I have assumed that issue was taken care of then.
This noise is not a clatter sound, but rather a feel that the engine does not fire perfectly consistently....as though a very slight pause was built into the firing order.
I guess it is time to chase the repair back through the last owner and see if there is a complete list of what work was done, and what exactly was replaced when the job was done.