well if your springs are beat you may have slapped the piston with that valve.any marks on top the piston?.i still dont think that noise is valves floating at that low rpm.this sounds strange just a couple simple things to check.you are checking timming off of #1 cylinder?.how old is the gas? any water in the fuel filter?

I agree with crypto, I don't believe it's valve related.

Several things cause pinging... too high of compression ratio for the chamber design... too lean of a mixture... too much advance on timing.... too hot of a spark plug (plug is glowing and preigniting the mixture) too long of a tip ( extended tip)... carbon build-up in the combustion chamber. I'd check on how hot the plug is first... sounds like it might be preignition. Also... and this may sound stupid... but check your firing order...at least you are eliminating the simple stuff. I knew one mechanic that chased a pinging sound on a v-12 for days...The car came to me.... I checked the firing order only to find two wires crossed...

Go to paint store and buy some toluene which is a primary ingredient in gasoline as well as what octane booster is made of. Add the toluene at 20% for effective knock control, if problem goes away it is a detonation issue. For verification search Home brew octane booster to read the article.

How did the top of the pistons look when you had the heads off?

I know you can blend xylene with gas and achieve the same result... xylene comes out of a crack tower a couple of steps up from gas and is 140 octane.... I've run straight xylene in an engine before... but it has such a low vapor pressure that you can only run it on a hot engine or it won't ignite.

I've tried 100 octane, dropped heat range on plugs, etc. etc. All the pistons look great, no nicks, no carbon, etc. My machinist suggests that I pull the motor, drop the pan, and check to be sure the crank is not real close to the windage tray. He says as parts flex, it may be hitting the tray causing the noise I'm hearing. Thanks for the suggestions, keep 'em coming If I ever figure it out I'll let everyone know.

Drew