We just moved into a new condo. Every 30.5 minutes there is a buzz that goes off - sounds like a dryer does when it finished. It does this every 30 minutes - 24 hours a day - 7 days a week. It is coming from behind the wall that is between the master bath and the utility room. We now from our experimenting it is not coming from the breaker box, it is not the a/c, not the hot water heater. Any ideas? The builder does not know but is thinking about it.

Cut an opening in the drywall to see what is inside. Easy to patch later.