I take my 99 XJR off the road over the winter to spare her the salt sprinkled so liberally over our upstate NY roads. I use a battery charger and start the old girl up occasionally and might even take her out for a run if the weather cooperates (which it hasn't the last two winters). Recently I've heard a "thump" at start up that repeats about once a second but goes away after about 10 seconds of idling. Yesterday I opened the hood, got my light ready, and visually inspected the idlers and belts to make sure nothing was off a groove. Everything looked good on all the pulleys and the back sides of both belts looked good so I jumped in an fired her up.
Sure enough the thump was there and while I was looking things over it almost looked as if the alternator was stalling for a second, causing the thump at the tensioner. Has anyone seen an alternator behave like that? I've had them stop charging, I've seen (heard) them make awful bearing noises before changing them out on other cars but never seen one stall momentarily. It seems to fix itself after a short time idling but I know from being stranded by one a long time ago that it's better to deal with these in the driveway than on the side of the road.
On a related note a quick look seemed to point out that the alternator isn't in an easy place to change. Let me know if there's something to watch out for when changing one out.
Thanks.

 

Probably not your issue, but when I first bought my 98 VDP (after it had been in a wreck), the alternator case was cracked. This caused alternator rotor to seize slightly inside the case, causing the pulley to be hard to turn. One way to check for this is to take off the belt and try to turn the alternator pulley by hand (it should turn easily).

A more likely cause in your case would be a loose belt/failing belt tensioner.

 

I redid the belts and bearings in the tensioners at the end of last summer. I'm pretty sure I spun all the pulleys (except the crank) by hand when I had the belts off. I don't remember noting anything odd about the alternator at that time but it could all be part of getting old(er). I do remember looking at the tensioner wear indicator and it looked ok. I definitely remember how tough it was to get the main accessory belt on, the tensioner seems to be up against a hard stop to get enough play to make the last pulley. It probably makes sense to check that again now that the belt has been on and running for a while. If I pull the belt I can check spinning the alternator by hand again just to know for sure.
Has anyone run into the need to replace the whole tensioner? Most of what I'd read here pointed to needing to pay attention to the bearings for the idlers.
Like I said after spending too much on a tow when an alternator failed on the NYS Thruway years ago I decided it wasn't worth it to ignore something when it raised it's hand in my driveway so I'll probably look seriously at replacing the lump anyway.

 

So shopping on line for the replacement alternator I noticed comments about "pulley not included" (not an exact quote). FCP shows the pulley removal tool and mentions it's for overrunning alternator pulleys. Could I be looking at an overrunning alternator pulley that's stopped overrunning, intermittently? It would suk to remove the alternator, replace my existing pulley on the new alternator, and run into the same problem. Has anyone had trouble with the alternator pulley itself?

 

The 3.2 and 4.0 non SC get a Denso alternator and I think the 4.0l SC has the same one. You can buy them cheaply for like €150 brand new. DAN672 is the Denso number and it comes WITH the correct pulley. A friend's 4.0l Souvereign and his 4.0l SC XKR do the same. When cold the tensioner will bang a couple of times. Always a couple of seconds after starting. After fitting new DAN672s they don't thump anymore. I THINK it is a regulator issue.

 

You may need to replace the tensioner. They go bad. An easy fix. Then, if the thump is still there . . .

Take the car to your neighborhood alternator and starter rebuilder, for about $175 and 30 minutes, he will fix the problem, labor included. My guess, your alternator bearings have frozen from rust and corrosion, but it could be a frozen regulator.