C230 AC Blower singing

[DMarble]

My 2000 C230 AC blower has recently started to sing when turned on. I thought it was a bearing problem but when I removed the blower and ran it with a straight DC source it is quiet all the way from 3 volts to 12 volts. If I hook it up through the Voltage Regulator then it starts to sing. More on the lower speed settings than on the high speeds. If I move the red wire up or down at the regulator the sound changes or disappears. I assume that this is a bad regulator but just want to get some verification before I spend the $175 for another one. Anyone have a similar problem or know how the regulator works to explain what is going wrong with it?