"The Hum"

Get your hearing checked. I'm sure I have tinnitus. I have a constant hissing noise that doesn't go away. (I have not been diagnosed though)
I hear a engine running noise at night. Only in my bedroom. I went outside one night to see if I could figure out where it comes from. As I neared the crest of the hill by my driveway I could hear it. A engine running for sure. Further investigation and I found out the railroad idles a few of their older locomotives all night. That was it!