SCD-4P52W makes noise?

[commodore_dude]

I just had DirecPath here to install an HR-21, and I was provided with a SCD-4P52W Dual Subscriber Unit to feed both tuners (no Mpeg4 hardware in my apartment complex yet sadly). I didn't notice until after the installer left, but the splitter "sings" at a very annoying frequency most of the time... it seems to change slightly when I change channels, goes away at times, and goes away completely when the receiver is unplugged. The splitter is powered by the coax cables, but appears to have a DC input as well. Is anyone familiar with this problem, and what can be done about it? I can't find much information on this splitter, on another forum someone posted about having the problem even after the splitter was replaced, but no one responded with suggestions...