Humming low sound along with sequencer

Please help. When I press start in my Ensoniq TS10 to record audio to Sonar4 Studio, the tracks start but, along with a low synthesizer sound, which goes away only when I press a different sequence on my Ensoniq. But every 4th or 5th try it's fine. What is the cause of this. Thank you.
Pentium D, 2.8 Echo Layla, Yamaha HS 50M, AKG 414...