This is normal ( but can be annoying). The convertor uses a switching regulator rather than a large heavy transformer to reduce the 110VAC to 12VDC. The regulator switches on and off at varying frequency to keep the voltage constant as the load changes. When there is very little load on the convertor the frequency is as low as it can go and it skips a pulse. This causes the frequency to get into the audible range. Higher load devices such as furnace etc, will not cause this. Usually caused by low wattage Incandescent bulbs or LED lighting. If it gets annoying, try turning on more lights, It will vary from convertor to convertor, If it is really bad the dealer may swap it out, but no guarantee it will be better.
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