excuse my ignorance for a while! I have a rattle in the rear of my XK, haven't checked it out yet but thought I could get a few ideas as to what it could be

 

Start with the easy stuff. Do you have stuff in the trunk/boot making the noise? Check in the spare tire area, plenty of metal in there to come loose.

Anything loose on the exhausts? Check the wheel inserts, mine rattle like crazy sometimes.

 

Thanks Mike, will do when I get back to it.

 

The factory rear shocks have an internal valve that can break off and rattle around inside the shock while the shock continues to function normally. Drove my wife crazy back in 2014 when it happened and it took some great advice from this forum to finally figure out the source of the noise (the right rear shock). Her 2006 XK8 is a convertible -- you could easily hear the rattle when driving with the top up, but not with the top down. New rear shocks fixed it....

 

I had a really annoying rattle in the rear and it turned out it was a little fabric clip that holds the boot lid liner to the boot. I just taped it over so it didn't rattle away.

 

I had a rattle/clunk in the rear forever. Super annoying and it turned out to be a coil spring insulator.

 

I just found a rear rattle that could only be heard with the top up. It sounded a bit like an exhaust hanger. I've been trying to track it down for months. I accidentally kicked the lower bumper and heard it for the first time outside the car.

It was a bracket that attaches the plastic lower rear bumper cover to the aluminium bumper beam. Just next to the tow hook cover. Easily accessible underneath.
The screw is tightened to a captive nut in the bracket, but the bumper cover must be removed to access the screw.
I just took 2 pieces of 1/2" x 1/2" of very thin scrap plastic, folded it in half, and wedged it between the bracket and beam. I'll check it in a month or so to see if it needs a drop of glue to secure it.

 

I have also had a sort of rattle/rumble for at least several years that I always ignored since I do not really hear it unless driving very slowly on a quiet street or moreso when rolling slowly through a parking structure. After reading this post I tried wiggling the rear bumper cover and I am pretty sure that is the sound. But looking underneath I can't see anything indicating where I would wedge plastic to resolve like you did. In fact I cannot see the bumper beam or anything similar since it is blocked by some plastic piece at the underside.