Sounds like you might have a broken timing chain guide, if it sounds like valve rattle just louder and only at idle. Rod bearings, wrist pins, or loose valves will all get worse when you rev up the motor. Timing chain will rattle like hell at idle but will quieten down when you rev it up. It could be piston slap but I've not had much experience with this , so I'm not sure what it sounds like. Pull the valve cover and look to see if the right side guide is broken.
