A running ultrasonic cleaner produces sound. Whether the lid is open or not, a sound that has a high pitch can be heard. The pitch of the sound fluctuates while degassing is ongoing. When degassing is done, the pitch becomes stable. When you open the lid of the ultrasonic cleaner while it is running, you will see many small waves. An ultrasonic cleaning solution does not become foamy. Therefore, do not expect to see foams when you look at the inside of the cleaner.