Funny clunk/rattle on 99 Yukon Denali

[99denali]

Hey, it's the same as a 98, so I hope I don't get any flames for posting about a 99!

Anyway, here is the story.

I just purchased a 99 Denali with 76,000 miles.

The truck rides fine, but for some reason, on uneven roads (wavy/small cracks). I hear this clunk/rattle sound that seems to be comming from the right side. It's very hard to explain this over the internet, but here it goes.

It seems like it's comming from the rear, but then again it could very well be the front.

The only thing that I can say, is when I had 3 of my friends in the truck last night, it hardly made the clunk/rattle sound.

I know it's hard, but can someone point me in the right direction? thanks a million!