Rattle during engine braking

You may have seen this already as I posted it to a Facebook group earlier when out and about in the car.

Driving to a friends earlier down an A road at 60MPH, I started to notice a metallic rattling noise as the car decelerates. Through a bit of experimentation, I could reproduce by accelerating the Jimny to 30MPH in 4th, and removing my foot from the accelerator. The rattle occurs during deceleration.

My friend was already tinkering with his Beetle when arrived so took a look under the car. Where you've got the drive shaft, and it goes from being thick at one end to thin at the other, the thin bit connects into a cylinder which is connected to the transfer box itself (I think, still learning this stuff). If you stick your finger in the bit where the thin bit goes into the cylinder you can get grease out. That grease has chunks of metal in it. Bad sign I'm guessing and possible the cause of the noise (it sounds like the right location when you're driving).

Driving home (on a slower road this time) the noise seems to have (mostly) gone. Perhaps the pulling out some of the metal bits has reduce the rattling/whirring noise.

Any ideas what this could be? The metal bits could be a red herring, but thought it worth mentioning.

Any metallic stuff sounds like bother. Sounds like the driveshaft is about to throw a 6. Don't leave it till it breaks as when they dd a lot of damage.
J999PRE is bound to have one

According a post elsewhere, the transfer box end of the prop shaft is a CV joint. Wonder if this is what's gone. Is the shaft easy to get off? Looking at the manual it seems to just require the removal of 4 bolts. I'm guessing it then just slides out and I'll be able to see if there is any damage. Also, should there be some sort of rubber boot over the end, because there isn't now...

Took a proper look, and it looks like the CV joint will come off by itself. Also searching elsewhere it looks like the joint should have a rubber gator over it.