My wife's 2004 Malibu LS V6 makes a metallic clunk or more appropriately a "tink" or "plunk" when accelerating heavily (like an expressway entrance ramp) just after shifting from 2nd to 3rd and then from 3rd to 4th. The noise is about one/two seconds after shifting. I crawled underneath to see if the heat shield was loose but all was tight. I hope it's not the tranny. Any ideas?