cruise speed valve rattle input.

I agree with Red, I doubt this is "valve rattle" but rather spark knock, otherwise known as pinging.

Did you have the head and or block deck milled, and is there some chance that either had previously been milled?

Until you map out the timing curve, you don't KNOW what it is advancing.

Too hot plugs, too hot engine, too much compression/ low octane gas, too much timing advance, or too lean of a fuel mixture