Strange Rattling

[NiRi]

I have a problem and I was wondering if anyone has a clue as to what it can be.

I recently purchased a 2004 Suzuki Forenza from a dealership, and I have a little concern. A couple of weeks after I left the lot with my car, I noticed a weird rattling noise on the right back side. It almost sounds like it's coming from the rear door, but I've slammed my door to make sure it was closed properly and it didn't stop. This mainly happens when I'm going a maximum of 45 miles an hour, and occurs whether I'm going over a bumb (slowly) or just driving down the road doing the speed limit. It could be rattling on the freeway as well, but I don't notice due to the noise from the road.

It's also pretty random. Has anyone else experienced this problem -- and if so, how did you fix it?