I can't answer about a 250. But I have a HTG 150w and it don't make a sound. So I wouldn't imagine a big dif between a 150 and a 250. I'm sure someone on here has a 250 from HTG that can chime in.
Sweet Awsome news. Thanks. I had an old hydrofarm 400watt ballast a few years ago that buzzed louder then a streetlight coming on.