Your probably hearing the tires on the road (I'm sure you know some tires can be noisy) the firewall has basically no insulation other than the 1/8" metal it is made from, so your going to hear a lot
If you want to try and get a more precise location of where the noise is coming from get someone else to drive the car, and listen front he passenger seat since that is the side where the noise is coming from, the motor has some whine to it on it's own so that is also a possibility
If you want to try and get a more precise location of where the noise is coming from get someone else to drive the car, and listen front he passenger seat since that is the side where the noise is coming from, the motor has some whine to it on it's own so that is also a possibility