Unnerving rattle

Any ideas?
My 110 (300tdi) has developed a rattle that seems to be coming from under my arse while driving.
It's not so bad while pulling away, but worse when engine braking / freewheeling, in neutral or with clutch fully depressed.
It's consistent with Road speed and sounds like I'm dragging a string of tin cans along.
I recently had a new clutch fitted, and I have changed the spider and bering cups at the rear of the front Prop.
I also noticed yesterday that when I select diff lock, the light on the dash comes on straight away, without driving forwards. I've tried reversing to disengage, and then selected diff lock again, light still comes on without driving forwards.
I'm about to get it up in the air and have a look underneath.
I'm going to concentrate on the props, handbrake drum and uj's.
Anyone got any other suggestions?
Cheers guys!!!

Prop UJ's can make all sorts of nasty noises - have a really good look/test of the front joint of the rear prop. I think/hope the diff lock light is a red herring. If the lever is able to move the actuating rod fully, the light will come on without driving.

Cheers Z, UJ's have a little bit of play so will change them.
Diff lock tested OK, chocked front wheels and jacked up rear left, main box in neutral, handbrake off. Wheel turned freely and wouldn't with diff lock selected.
My other concern is that there seems to be some play at the rear diff flange if I push / pull the rear shaft up / down, left / right (not rotational).
Is there a bering in there that I can change without removing the diff?
Cheers again!

May sound silly, but have you greased all the UJ's

Cheers

Exhaust loose? Tapping on the gearbox cross member?

[QUOTE="I have a nut leftover, post: 3893568, member: 9165"
My other concern is that there seems to be some play at the rear diff flange if I push / pull the rear shaft up / down, left / right (not rotational).
Is there a bering in there that I can change without removing the diff?
Cheers again![/QUOTE]
Yes there is a bearing in there, and it sounds from your description as if its shot. The only real way of changing the bearing is to remove the diff, its not too bad on the rear as you only need to pull the drive shafts out enough to clear before taking the diff off, the front is a lot more work. I struggled for ages trying to get the bearing out of my front without removing the diff, ended up pulling the diff anyway. That was a day wasted.

I am sure it won't be this on yours, nut l had exactly the same rattle on my TD5. It turned out to be a metal flange at the rear of the prop where it's mounted to the rear diff.
As the noise was so unnerving l let the dealer look at it, they diagnosed a loose "flinger" and charged me £100 to basically remove it "you don't need it" they said.