Sometimes I hear a throaty droning sound as we get between 2800-3100rpm in most gears. It's really hard to place the sound. It sounds like the engine, but of course could be exhaust transferred forward... or who knows. It doesn't sound bad, and I don't always hear it. Interesting.

S model

 

It's normal. If you were to install a stiffer lower mount such as the BSH the whole car vibrates at 2800rpm. With the stock mount the drone is really subtle. I wouldn't worry about it.