HELP! My car is doing something weird!?

I have a 2000 Nissan Maxima with 98,000 miles on it. It drives really nice and smoothly on level ground, but whenever there's any bump (even a small bump) in the road, I hear a rattling/thumping sound coming from the front end. It sounds like it's by the left front tire, and it's scaring me. I keep thinking the tire is loose and will fall off when I'm driving. Somebody told me it could be the shocks, but the front end of the car doesn't bounce. If somebody could give me a clue as to what the problem is -- and approximately what it would cost to fix it -- I'd really appreciate it.