Is it only at low RPMs (just off idle) in 1st or 2nd gear, or did you also hear it idling in neutral? These cars do not have dual-mass flywheels, on top of the fact that the S has a lightened flywheel, which means you can get a little gear rattle/noise in low gears at very low RPMs. That may be what you are hearing if it goes away as the revs increase. Someone can correct me if I'm wrong, and noises are hard to diagnose over the internet, but it was just a thought.
'14 Evora S - Chrome Orange/Ebony
'02 M5 - Carbon Black/Silverstone
'01 M3 - Imola Red/Black (gone but not forgotten)
'08 RS4 - Imola Yellow/Black (gone but not forgotten)