Nails in a Coffee can noise when letting off my clutch?

What is your question?

i noticed this noise when i would let go of the clutch it would make a sound like putting some nails in a coffee can and shaking them up. It lasts for a few seconds the stops it doesn't always do this. Also i have been noticing it while im driving at low rpms. it doesn't always do this but it does last longer i don't notice any performance changes and shifting is just fine. i'm paranoid hoping its not my transmission or anything major of course everyone hopes that. i have heard many different things from people just to wonder the heck that even does. It would be helpful to have a checklist of things that i should do because i know little about engines or vehicles im trying to learn everything i possibly can.