> What do you think is its source? Really good question. I think it can be a combination of vehicle/road/highway noise, industrial noise, power plant/turbine n

> What do you think is its source? Really good question. I think it can be a combination of vehicle/road/highway noise, industrial noise, power plant/turbine noise, aircraft noise, and who knows what else. It seems hard to localize by ear, and the actual source could be miles away. In some cities I notice buzzing electrical transformer noise which isn't quite low frequency rumble but it is still annoying. There are probably other bizarre and mysterious sources of noise - I recall a story of residents of Sunnyvale (?) being disturbed by Apple's underground vehicle testing site which apparently operated at night. Some companies and research labs do seem to run loud equipment and engines at night, I imagine because it would be too loud for employees who work there during the day. ;-)