The RCL-10, as many other Boss units, are very sensitive to the power. Of you have any sorts of ripples in the power supply, this will be immediately obvious as a loud hum. Any cheap 9V powersupply that you buy out there will cause a hum. Yes, really.
Make sure you are using a Boss adapter, or another high quality power supply. I have only two that work with mine without humming, the original Boss one, and this one.