Dave: I was about your age when the HUM started for me....very similar to what you are experiencing....I would notice it if I was taking a short nap in my bedroom and I too looked out the window thinking it was a truck or vehicle parked outside! I am not sure if you have any hearing loss but it progressively got worse for me as I aged.Now being 63 I have profound hearing loss and hear the HUM all the time. Mine gets worse with noise. When I wake up in the morning it is probably the quietest but as the day progresses it get worse. I have been to tinnitus specialists and they recommend sound therapy which I can now do through my hearing aids. I use a mix of brown noise and river streams...that seems to relax me and makes it less noticeable. Have you worked in loud environments? One thing I notice is since I have been retired the HUM reminds me of the environment I was in...I had an office above the utilities area and perceive the running motors and compressors as the HUM I hear now! Strange! I have found out that everyone experiences something a little different from person to person....hope this makes sense!
Scott