Pinging sound. If it is pinging then timing is slightly too far advanced. Try retarding the timing and see if the detonation sound/pinging sound goes away.
At idle, I run my automatic 440 at 5 degrees BTDC. The 1969 service manual states 7.5 degrees BTDC, but the engine starts to knock going up a hill with modern gas. I used to use 91 octane, but now I mix 91 and 100 to get 93 octane.
