When it's quiet and when I'm about to go to sleep, I hear really low pitched ringing in my ears. Is this normal?

Perhaps. It is not unusual to hear various noises in the ears especially when there is no background noise to mask the ringing. Oftentimes, this is associated with some hearing loss, so it may be worthwhile to have a comprehensive audiogram (hearing test) done.
Not but commo. This is tinnitus... Not normal but very common. You should see your local ENT and get your hearing checked.