Rattle / knock from rear

I’ve got a metallic rattle/ knock coming from the underside of my defender. It sounds like the “dunk-dunk” you get from when you run over a loose manhole.
And no, I’m not running over loose manholes!

2 weeks ago it was sporadic. Happened when I went over bumps, but not every time and it didn’t matter if it was a sharp hit or a soft one. But the last 2 days its been happening a lot more. Its every bump I go over now.

It seems to be coming from the rear or midsection but its hard to tell as I’m driving when it happens
I’ve had a look underneath but everything looks fine. The shocks and springs seem solid, the exhaust doesn’t move any more than normal, the fuel tank isn’t budging, the prop shafts are solid! I pulled on the corners and they rose back up normally.. I’m at a loss as to what could be making the noise!

Anyone got any ideas? I’m off to Wales this weekend and I’d rather not be in the middle of some sodding field on the Brecon Beacons when something falls off!!

A frame ?

Mine did this after fitting a lift kit and it was the propshafts. Basically front one had broken so I replaced it but the sound was still there but nowhere near as bad. I'm now about to change the rear prop too as I thing the UJ's may be binding up.