I would suspect railroad traffic sequenced along a track. During the night= when other traffic is quiet you may see the railroad from farther distance= .. In my location the noise builds and then decays over several minutes. I= don=E2=80=99t see one every hour but there is a daily schedule that shows = up and when the wind is right I hear the whistle to verify. =20 =20 Hi Randy, From this I assume that you still have steam trains with very heavy eng= ines, in the US? We are mostly electrified with motors in each carriage, bu= t there are some local Diesel trains. We don't often see train signals, unl= ess we are very close to a line. Regards, ChrisI would suspect railroad traffic sequenced alon= g a track. During the night when other traffic is quiet you may see t= he railroad from farther distance. In my location the noise builds an= d then decays over several minutes. I don=E2=80=99t see one every hou= r but there is a daily schedule that shows up and when the wind is right I = hear the whistle to verify.Hi Randy,From this I assume that you still h= ave steam trains with very heavy engines, in the US? We are mostly electrif= ied with motors in each carriage, but there are some local Diesel trains. W= e don't often see train signals, unless we are very close to a line.=Regards,Chris