Rep Power: 0

I have an '04 civic with 98000 miles and 5 speed manual. Yesterday I started my car to leave work, everything sounded fine. As I arrived home I noticed a rattling sound coming from the engine. Not a metal rattling sound but something similar to a plastic rattle. Checked all plastic, everything was tight. It only does it when the cars been driven for a while and it only does it between 1800-2200 rpm. I put a screwdriver on the valve cover while my cousin revved it to 2000 rpm and it didn't sound like it was coming from there. I started the car this morning and everything sounded fine. When I got the work, I heard it again. Any ideas? Thanks

 
Rep Power: 0

same rpms in all gears?

 
Rep Power: 0

Yes but it comes and goes. Somedays it is louder than others. I've listened to rod bearing videos, etc but it doesn't sound like that. I took it to a mechanic he put a stethoscope to the engine said he really couldn't pinpoint it it could just be noisy valves