
Indeed.Most likely lighting contactors as they come on around sundown.
You are not going to like the answer. That's normal. It's what switchboards do. The only thing fixable might be loose equipment or covers which may rattle or resonate, but those would create more of a buzzing or rattling sound. The hum and growl are just the equipment doing its job.It's the left box... the whole thing is vibrating and making noise....
Have you ever been near any of that equipment? Press your ear to even a lightly loaded home panelboard and you'll plainly hear it humming away. No transformer required. Turn on significant loads and you'll hear it farther away. Higher current equipment hums even more loudly. It's something unavoidable with AC.I've never heard an electrical room, switchgear, or panel boards hum, even with "loose connections", without a transformer or contactors present