Banging noise?

[1978 RD400]

i have a 78 rd400 and when it's idling, it makes this banging noise.
when i turn the throttle it goes away. does anyone know what's wrong and how to fix it?

Thanx

[premiumjo]

Piston slap maybe? Mine has some rattle when you let off the throttle, but it's pretty quite at idle. It has quite a few miles on the top end, but it was nice and quiet after the pistons/rings/bore were new. How does it run otherwise?

[RD400]

I would get a wooden dowel about 1 inch in diameter and 2 ft long and touch it to various points on the motor while holding the other end to an ear. You can almost pin point the location of the noise this way because the stick will transmit the noise directly from the location on the motor where you're touching it to your ear.

Metal also works well but gets hot quicker.