Hear something metal rattling

I have a 2000 Honda Civic EX with approximately 63,000 miles. Recently, I have heard something metal rattling/vibrating. The rattling noise doesn't always occurs at first. It seems like you have to be driving for a few minutes before the noise appears.

I took it to my local mechanic and they did not hear the problem so they can't diagnose it. They said they took it for a test drive but probably not long enough. However, I still hear it after driving for awhile. It just sounds like something metal is shaking below me. Sometimes when I'm idle at a red light, the noise occurs and then dies. When I hit the gas once the light turn greens, the noise gets a little louder but then dies down.

I can't seem to figure out what it could be since it doesn't always happen at first. Why would it only rattle after driving for 5 minutes? It may just be something minor but I woudn't know unless I can get someone to hear it.