Yo Jfrech,
The interference you are hearing is caused by a strong RF (radio frequency) signal that is finding its way into your phono stage, cartridge or cabling. Start by unplugging everything. Disconnect the phono stage from the preamp, the phono cable from the turntable and from the phono stage, and then turn on the just the preamp and amp. Listen for the offending interference on the input you use for phono. Next turn down the volume and hook up the interconnect from the phono stage. Turn up the volume and listen again. Then connect the phono stage to the interconnect. Listen again. Keep adding pieces until you find the one that is allowing the interference to get in. Once you find your "problem-child" component you can try three things:
Shield, re-orient, or replace.
Shielding means that you apply copper foil, mu-metal, or sheets of CRS (cold rolled steel) around the component to keep out the RF. Cables and cartridges can act like antennas and sometimes if you simply move them to another orientation or location, that will greatly reduce the RF problem. In the event that the problem is a cable, try a longer or shorter one. If all this fails, you can throw new components at the problem until it goes away.