If the noise is when you lightly get on and off the gas at cruising speed (usually around 1700 - 1800 rpms), you hearing the transition from 8cy to 4cy. There's a change in exhaust tone as soon as you gently give the car gas. That's the AFM.
If you get an aftermarket exhaust it will be much more noticeable. A lot of guys are getting their L99's tuned to turn off the AFM.
Also, as suggested, if your L99 is running slow, you should try to fuse pull mod. Because "normal" running L99's are fast. I've seen bone stock L99's run low 13s in 90 degree heat.
And the fastest stock 2010 Camaro over at Camaro5 is an L99 (12.86 in the 1/4 mile).
2010 Camaro 2SS/RS, L99 A6
Bolt ons (12.10 @ 113.60 mph, 1.77 sec 60')