I have a 3.5 v6 and brought it into the dealer for the same reason, they told me that it was just the fuel injectors making the noise and that it was normal...I've heard this same noise coming from both diesels and new cars that have direct injection systems, but this is the first car i've had that has made this noise from a normal fuel injection system, I don't hear it all the time, just occassionally...I guess its just GM's design..
2007 G6 GT
Debadged, K&N Drop In Air Filter, Pioneer DEH-6200bt w/ Metra Kit, L.E.D. Tailights, L.E.D. 3rd Brake Light, 7500k High beam, Low Beam, and Fog Lights, L.E.D Lights Behind the Grille, L.E.D. Reverse Lamps, Ya, Basically L.E.D.s all over, And more to come....