From your description, It sounds like detonation to me.
Not a good thing for continued life of your engine..
Does the noise sound like a bucket full of marbles when throttled under load..(pinging)? If so it's detonation.
And if so, try retarding your spark a bit and see if it goes away..test by going up on the Freeway and hammer it, If no noise, that is the problem..also try pulling a steep hill in the highest gear you can at the lowest speed, if it's detonating, it will ping.
If it gets better but won't go away, get some octane boost , double the amount, and try again..if it goes away, use a better grade of fuel.
If it still pings, try "Fattening " up the mixture a bit.
When done, recheck the timing, and test one last time. If it pings after that , you have carbon Hot spots on the pistons, you need to run a cleaner through the induction system for that.