I live in the suburban mid-south, which means between March and November someone's AC, attic fan or whole-house fan is running in my neighborhood. During the ARRL Field Day we had a power outage that crippled the whole neighborhood. Luckily I have backup DC power for the station and laptop. Normally, 15M has S7-S9 semi-pure white noise. By semi-pure I mean it isn't just 60Hz+harmonics hum. That day, noise levels were S0. Not just on 15M either. Should I be looking for a distinct noise source or is this normal for a suburban environment?