If the inverter contains a transformer (And many (I think all) do) or if the converter does (Likewise, in fact agaom I think they all do).
"Large" power transformers are usually made with an Iron Core that consists of layer afater layer of "E" shaped leaves. with the windings usually around the center part of the "E". these leaves are held together by a type of lacquer. Over time the lacqure fails and the leaves vibrate as current flows through the windings, which are, after all, electro magnets. The result is HUM and the volume is porportional to the current flowing.
Home is where I park it!