There is a loud vibrating, humming, drilling noise in several walls in my home when the sprinkler system is on. The sprinklers are scheduled to run at night, so I wake up to this noise. It lasts about 20-30 minutes but it doesn't happen every time the sprinklers run. It can only be heard in my bedroom and my children's room. It is highly irritating. What could it be and how can I get it to stop, other than not running my sprinklers?