I think I heard the same noise (for the same duration of time as it usually does too) the other day at around 30-40mph. I think I was letting off the accelerator pedal but can't be sure.

 

The sound I get is more off a high pitch buzz/beep. It's fairly pronounced even when the radio is on. It occurs usually during moderate acceleration or braking. If you want to reproduce it, just put your car in reverse and lightly release then apply the brakes, you'll get the sound. I took it to the dealer and they were able to reproduce it but insist that it's "normal". Just because its a hybrid doesn't mean that weird noises are normal.

 

I've heard it's the CVT before though it really has an electric whirrrr to it (I've got a 2003). I've never noticed the sound in my Insight (5spd). Anyone with a stick - 03-05 - hear it before?