This is quite nice but there is some constant mechanical background noise which is quite annoying. Is this a bug or intentional?
That sounds like a bug, the only mechanical background noise should be the conveyor belt when it's moving, and also an alarm sound if you have gems in the danger zone.
The game autosaves after each turn, so you could try refreshing the page and see if it goes away?
There are volume controls in the top right for now, or failing that if it's a truly stray sound then you could mute the tab directly in your browser by right clicking it.
If the sound is persisting for you after a refresh can you let me know, along with any information about when it happens?