Wow...that's a new one, and I have to say...I've never heard it (but I usually have my audio system on 100% of the time as well). I'll listen for this over the next few days.
One thing I *do* hear is a low-pitched, cyclical 'droning' sound from the engine bay at exactly 52-57mph. Any slower or faster, and you won't hear it.
It's not unique to the hybrid, because my coupe had it as well. But I'll be darned if I can figure out what it is. And here's the funniest thing - I don't think it was there the first few days I had the car, but popped up about a week or two later.
