At the buss bar that connects the row of 12V DC mini-circuit breakers, when plugged into shore power, you should read between 13.2 and 13.6V DC. That's if the Power Converter is working. In this picture, one of the thick wires on top connects to the Power Converter. If you put the red lead of your meter on that wire, and the black lead on a known good ground, or on the negative battery terminal, you should read 13.2 minimum. If you read something like 11.8V, you're reading the battery output and it's too low to be usable. A low reading would also indicate the Power Converter is not working. In that case, you'd need to determine why the Power Converter is not providing power to the buss bar.
Note that your arrangement of breakers will be different and the orientation could be vertical. Look for the thick wires. Measure on the side of the breakers that doesn't have the copper buss bar.
The noise you're hearing could be from the transfer switch and one of several possible reasons the Power Converter might not work is that the transfer switch might not be sending power to both power legs. In that case, some of the other 120V AC devices would be dead.
It's also possible the prior owner kludged things up when he put in the solar system. If the Solar panel and charge controller are sending power to the batteries all the time, it's going to confuse your diagnostics. If he put in an inverter to provide 120V AC from the batteries, there are many ways to do that. Some work well. Others not so much.
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