Grussell hit the nail on the head. The bushing are likely either worn down to a nub, or worn away alltogether. When you look in the track on both sides, you can clearly see where the play is. The proper fix is to remove the headliner, then the sunroof assembly and then install new bushings, but that's a real pain in the ***. I simply sized up some rubber o-rings and slipped them over the nub and pressed them into the track. Took about 10 minutes last summer and I haven't heard a clackety clack since. Works like a charm. The rubber is better than the original plastic imo. Just remember to lube em up with some white lithium grease after installing them, and your noisy sunroof should be a thing of the past...

 

I am a big fan of A/C (Texas...), but mentioning fuel economy on a Discovery... really???
Get any other car/truck and it will save you MPG on the highway!

 

Ive always been a fan of keeping them closed. I had them open one time when i decided to go through a waterhole... Ya it did just what you think. Came in the sunroof and doused me. Awesome! Ive been wanting to make a T-top out of the front one haha.

 

I know it been a long time but i cant find much info about these bushes. Where exactly are they supposed to be please?