A few days ago our Bosch dishwasher started making a low buzzing sound, but kept running fine. We checked the bottom of the drain area for debris but didn't find anything. Yesterday when it reached a drain cycle, it started make an awful racket that sounded like a siren, REALLY loud. We opened the door to stop it and turned it off. From what I've read, it could be the circulation pump, drain pump, or drain valve. How can we tell which one is the problem? The water was mostly drained when we opened the door.