HID Ballast Humming

I have one of the 85/65/45 watt HID flashlights from e-bay and I purchased a replacement bulb/ballst combo from e-bay and after about 10 minutes the ballast developed a fairly loud buzzing sound. So is this normal or is there something wrong with this ballast as it never did this before the whole time I have owned this light when it had the original bulb/ballst in it. Also I have a friend of mine who has the same light with the same bulb/ballst in it and his does not make that buzzing sound so I am wondering if I have a defective ballst or bulb. Any help or input would be greatly appreciated. Thank you.