Obviously front suspension noise is a common issue with these older Jaguar models. You can include XK8 models of similar years to my info here as I found the issues are very similar. Let me start saying, I personally owned a bunch of XJ's and Xk's all the way from model year 98 up to '14. On top of that Range Rovers. That is simply because I like those. Now having said that, I am maintaining and fixing these myself as dealer prices are so prohibitive that it won't make sense. Now a quick guide to front suspension noise on the 98 to 03 XJ8 and similar XK8 series. I am talking about a mileage range of 80k to 160k on the cars I am writing about here.
1. Of course look for the obvious. Meaning lift up the car, take wheels off, do a very thorough visual inspection, try to move things (prybar) ect. Many issues may present themselves. You may find leaky shocks, rotten bushings and so forth. Start with repairing the obvious. However, just as often, it is very hard or impossible to spot the culprit. So my approach is starting with the most common, but cheapest and easiest.
1. STABILIZER BAR LINKS: (aka sway bar) the stabilizer bar links are often suffering from rotted and worn out bushings. Start with those. Its an easy job. Took me $ 50.00 online and 1 hour for both sides (I ALWAYS do boths sides for cheap easy stuff). Not as common but while you at it. I recommend replacing the STABILIZER BAR MOUNTING BUSHINGS. $ 30.00 online and takes about 30 minutes.
2. UPPER SHOCK MOUNTS: Very easy to do. The OEM orange thingies are often dry rotted or worn out. Upper shock mount towers are removable by removing 5 bolts. On drivers side remove mounts of steering fluid reservoir to get to and move out of way. Unbolt upper shock on the center nut. Cost approx. $ 65.00 a pair for the aftermarket stiffer black ones. Time max. 1 hour.
All the above I would rate skill level: Easy
3. Check the SHOCKS themselves. Often I found bad shocks and/or bad LOWER SHOCK MOUNTS. More often a combination of both. I usually don't bother replacing the lower shock mount bushing UNLESS I know for sure the shocks are fairly new and good. I believe in most cases once those mounts are gone the shocks are ready to go as well. And in ALL the cases I did it, I was able to confirm at least 1 dead shock. (meaning no pressure). Replacing 2 shocks means about 1 - 2 hrs. truly depending on how you work along and how you are setup. Took me a leisurely 1.5 hrs. Cost of the shocks range from as low as $ 75.00 a pari for cheap Chinese ones. (good enough if you don't plan to put more then 15k miles on them) to some midrange aftermarket at $ 280.00 a pair to OEM which are about $ 220 each. All the above WIHTOUT electronic damper control. If you have electronic dampers control you need to go with OEM at over $ 300 a piece. Work is about the same time.
4. STEERING: Here I mean tie rod ends and steering rack. Often I find a loose TIE ROD END. Its almost always the outer. $ 30.00 each. Time about 45 min each (if you have a good setup and tools) and may be 1 hours for both. You can easily spot those by having someone pull the steering wheel back and forth and watch the ball joints. Next common thing on steering is the STEERING COLUMS components itself. There is a TSB as there are some linkage that start to get play on some car ranges. Look around on the forum. Its here. But I never found that problem on my cars to be there at all or rarely occurring to make a noise. Also check the STEERING RACK MOUNTS and the STEERING RACK itself. I do hear there are occasional issues. I never had that happen on any of my cars though.
FRONT SUSPENSION COMPONENTS: Yes, these often like to wear out. On many of these cars as early as after the first 20k miles. I had that happen in 2 cases. However, once fixed the replacement parts seem to last much longer. But here are the most common items I find: LOWER CONTROL ARM BALL JOINT and or BUSHINGS. Lift the wheel up and once of the ground push down on it or use a prybar to check for play. Also check for busted rubber covers. If you find play they WILL make noise. These are not expensive. About $ 40.00 each online. But they some may find the job difficult. Many replace the complete lower arm. Lots of work. Since its a common issue here is my suggestion: If the mounting bushings are not rotted and still good, leave the control arm in the car I found out you CAN press out and press in the new ball joint on the car with the right tools and good skill. It is a bit of work as you have to remove a bunch of pieces i.e. rotor, caliper ect. ect. But removing the control arm means using a special tool spring tensioner ect. That however is NOT a job for the faint of heart with no experience in these things. UPPER CONTROL ARM BALL JOINT: Not as common but I have seen it. In that case I would just replace the entire upper control arm with bushings as it is not as labor intensive as doing the lower.
Last: on 2 of mine I found a worn front wheel bearing. Easy to diagnose. Lift wheel of ground and wiggle to see if there is play. Replacing these again is labor intensive. You need to remove the steering knuckle aka wheel carrier aka vertical link (depending where you live). You need to get instructions (ALLDATA) at least that what I did, and just follow those. Need a press and a full set of mechanic tools. Again, not a beginners job.

Not to forget I hear about engine mount issues. I never had it happen. That is easy to spot if you watch the engine in gear while adding torque. It will jump around a lot. Again, I never had a bad engine mount on mine even after I twisted the hell out of one while removing/installing transmission. Same goes for subframe mounts. I never seen those so bad that they caused major noise or movement but I hear about it happening.

I hope this somewhat compiled info helps some of you. It is not complete nor is it meant to be. Nor do I know all of it. This is what I experienced with my own 4 Jags 98 XJ8 VDP, 97XK8, 2000 XJ8 base, and a 2003 XK8. All between 80k and 160k miles on them. And I could see a common pattern.

Oh, one more tricky one: 2000 XJ8 heatshield above driver side cat converter: Loose, deformed and/or not properly installed. Every time engine moved when hitting pothole our major bump the heatshield hit the frame rail. That took me a while to figure out and I never heard anywhere else about this one. Happy noise hunting. The reward is a quietly purring cat.
W

 

I have a 1995 VDP.....Absolutely love the car.....Started developing a suspension. "knocking", over the past year.......I have thrown $1200 at it, changing shocks, sway bar bushings, sway bar bushing ends, upper and lower control arm bushings, tie rod ends.....Took it to four different shops.....To find out on my own, that it was the lower shock mount bushing, on the car, not the shock........$10 bushing....I am both happy and very mad at the same time.......The shops were recommending me spend another $1000.00 to chase it down......If any of you folks ever have this problem, jack the vehicle up, get the A frame and brake disk up in the air, then lower it on a jack stand......Twist the bottom of the shock.......If it moves.....There is your problem...It will not move unless you lower it on a jack stand.......I love my Jag....Driving anything else is uncivilized......It no longer sounds ghetto going down the road......These mechanics do not give a darn about fixing your car, and will let you spend everything you will pay them......And I bet a dollar to a doughnut, they would never have figured it out.........

 

Nice catch. I suspected those. too. I found that even if the lower shock bushings look fine visually but are loose in the bracket they make noise. What I mean, I tightened the attachment bolt on the lower, so that the bracket squeezed tight so the lower shock mount can move back and forth on the bolt. I found mine had about 3/16" inch of play between the bracket and the bolt is loose inside the sleeve. I can only guess that when they are new there is more rubber on the sides. \
And regards to the dealers you are correct. When it comes to noise they don't get it. I had same problems with Range Rover at dealership. I even got them on the dashcam talking trash when test driving not getting anything. Only thing in their defense I can say: They only have your car during a very limited time and not much time to actually work on it. We DIY guys can tinker on and on and test drive in different conditions in our own garage until we find it.

 

I have developing squeaks diagonally on my undercarriage. From my research the bushings can be quieted down with a good spray.
Seems to me the home remedy is a AT sealer, that is sprayable. Lucas's doesn't spray. AT fluids don't eat rubber in general, and the sealer could have an added effect.
Did the rear. Very interesting. I need a Bulgarian boot on the long control arm. So far the rear is quiet. Lots of bushings there. Wow.
Next, to the front, when I have the energy. There I have a squeak, and a click/click.

 

New shocks, upper shock mounts, boots, and bump stops (which had completely disappeared from my car) eliminated all the front clunk, as did new shocks and coil spring insulators in the rear.