Buzzing Sound (engine compartment or firewall area?

I drive and reach 40-50mph this buzzing sound starts and it sounds like static/buzzing, it's not coming from the speakers but from the engine compartment/firewall. When I hit the brakes to slow down the sounds lowers till I can't hear it anymore, then when I increase acceleration (45-50mph the buzzing sounds starts again and get really loud the faster I go