Thanks for all of your quick replies.
The first time I noticed this irritating chime, my new pup was riding in the passenger seat and when I figured out what was causing the irritating noise, I buckled the passenger seatbelt. Noise stopped, but pup started chewing on the belt. I then stretched the belt behind the seat and locked it. Worked fine until I had a real person passenger.
A few weeks later of locking and un-locking the passenger belt behind the seat, I had the bright idea of cutting the wire to the passenger belt lock. That makes the chime stay on. Don't do this!
I then talked to the guy at the Subaru service desk. He told me about the thing of locking and un-locking the belt 15 times, but he said that would only disable the chime for one time.
On the way home, I also had the idea of getting an extra buckle, and thought if there was a center belt in the back seat, I could cut it off and use it. I couldn't find a center buckle. There are 3 latches, but only 2 buckles.
We're having a little of that white Montana winter sunshine right now, so when it stops, I'll try the 15-20 buckle/un-buckle thing. If that doesn't work, a trip to the junkyard will be in order.
Again, thanks for your suggestions.