We paid about $100 for our Weiser programable lock about 4 years ago. Our bedroom is 2 rooms (or one door) from the front door. We can hear the guests hit the code buttons. If they mess up (or the door messes up) we can hear that, too, and rescue the guests. After a couple of failed attempts, the high pitched alarm goes off for about 5-10 seconds, indicating failure.Ours is made by Weiser Lock. There is no attached instruction on how to reprogram the code anywhere on the outside of the unit.
As Joey mentioned, the unlocking mechanism slows down as the battery life dwindles indicating when it's time to change the battery. If it doesn't get changed, the deadbolt will only retract halfway and the door won't open.
As for noise, you probably wouldn't hear it operating if you're in an adjacent room with the doors closed.
Overall, I'm happy with this unit. It was on sale for an excellent price from Costco (can't quite remember exactly what it was)..
We have two codes. 1) our personal code what doesn't change; and 2) the guest code that we can change at will.
When it is cold, sometimes the guests may have to enter the code at least twice. The other night, it didn't retract all the way and I had to intervene.