I know- another post on vibration, but I can't seem to find one that best describes my Freelie's ailment...

It's not a permanent thing, but sometimes when I drive on main roads, I can feel a rumbling/shaking through the car if I hold a steady speed of 50mph after about 15 minutes of driving.
It does this for a while (say, a mile or so) and then drives fine again.

I recently had a seized front caliper replaced so I know it's not that, although it is reminiscent of a brake getting hot and swelling/dragging but in that instance, it would stay like it until I slowed down/stopped and I could feel it through the brake pedal as I did so.
But this vibration just does its own thing and disappears, but it's bothering me because I've noticed it happening occasionally- and I drive on the same roads regularly so know it's not road surfaces etc.

Changing from 6th to 5th gear doesn't affect it, so it's not engine labouring.

The handbrake hasn't started sticking (yet- although this happened before and led to similar symptoms with the brakes getting hot).

What else could it be?

(It's a 2007 2.2TD4 160ps with 256,000 miles on it- I've had it seven years!)  "We're in danger, we've got to get out."
"What are you talking about?"
"I've been called a ponce."

Have you tried removing the haldex fuse

The rear drum brake linings have been known to detach so you’d get the vibration of the sticking handbrake that would go away as they wear only to return when they move. It’s certainly worth a look.