nothing wrong with it... you could run a higher octane fuel and it might go away. Or, you can adjust timing to get rid of it, but your mpg and/or performance would probably suffer. What you're hearing is known as 'pinging'... search that and you'll find tons of info
this is the V6 forum too, you might have better luck in the GT ones.
edit: i just read it's only on startup... in that case, i wouldn't worry about it