The loud buzz is usually because the Ultrasound driver fire at about 25Khz which itself is silent, but the vibration it creates in the metal tank results in a lot of harmonic resonance - visible as ripples on the surface of the water.
If you look at the edges of the tank you'll likely find a dense rubbery seal which helps to dampen the harmonics.
Larger machines are still unpleasant to be around though. Sadly the really quiet machines are incredibly expensive as they operate at far higher frequencies.