It's normal, the HID Ballasts are electronic and use switch mode power supplies to convert DC from the battery into AC for the Arc Lamps as well as high voltage for the arc. The noise you hear is the ballasts switching on and off hundreds of times a second to create the AC at 87vAC during normal operation and up to 22,000vAC to ignite the lamp when it is first turned on.
We get a similar noise from our three phase ballasts that we use to power our followspots at work. Of course...those are powering 1200 watt, 110,000 Lumen output lamps that we use to throw light a hundred feet, and those are considered small in our industry. :lol: They are also much louder. I have personally replaced 10,000 watt Xenon arc lamps in larger fixtures...you know, the kind you have to wear a kevlar suit and gloves when you change it.
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