Audio Noise Question

I just got my Optura 400 and while playing with it (reading the manual etc - yes I DO read the manual) I noticed lots of motor noise when in auto mode. It was a quiet room. I switched to manual mode and lowered the mike sensitivity and the motor noise went away.

Question: what does auto mode do for audio levels? In a more normal (i.e., non quiet) environment, will auto mode lower the mike sensitivity and therefore eliminate the motor noise, or should I plan on keeping the camera on manual?

Thanks - doug