rattle or slight knocking noise in engine

Now that i have the brakes fixed i'm gonna move on to the next problem. I have a noise in the engine area that i can only hear at or around 1500 rpms thru the 2000 rpm range. It sounds like a piston rattle but only at those rpms. When it gets above 2500 rpms it dosent rattle anymore and it dosent rattle at idle either,dosent rattle at start-up. i dont think this is doing any damage to the engine ,there is no metal in oil . just wondering if anyone has had this problem or has herd of this problem. I read somewhere the flywheel might be cracked or it might be the torque converter, i just dont know . I thought i might try to get some of these problems taken care of while everything is slow and i had some extra time. :shrug: