Strange sounds Seattle this morning 5:30am

I just was woken up by hearing what sounded almost like a train laying on their horn for 10 minutes straight. I know it wasn't because the sound would get louder and then quieter and had a humming sound to it. It sounded almost like when you take your finger and run it around the top of a bottle. Sound was so loud it woke me and my fiance up, also the birds outside were going berserk. Did anyone else hear it?!!

Was planning on sleeping in today but I guess I am up now!