Switching from Adobe Audition to Sonar X2 - buzzing sounds

Sorry - newbie question. Coming from Adobe Audition background. Working on a Win 7 64 bit PC with plenty of processing and memory.  Have successfully mixed with Audtion for more than 3 years. Recently had the urge to get into music making. Bought a MIDI keyboard (Novation Impulse 61) and a brand new copy of SONAR X2 Producer edition.  Before I can even hook up my keyboard, I decided to do a simple recording with SONAR. Same old gear as am using with Audition - the audio interface is through a Mackie PX12 audtio mixer. Microphone is a cheap Behringer condensor one.
 
Unfortunately, I am getting a loud buzz from my basic recording on SONAR. I believe I have the input / output correctly set up (via the soundcard).  The recording appears to be happening correctly but on playback I hear a HUGE buzzing noise. I route my PC output via a Behringer Headphone distribution amp. Regardless of whether I listen using my headphone or via the KRK monitors, the buzzing noise is always there. Switching to Audition and repeating the same steps does not present the buzzing noise.
 
Clearly there is something very basic I am doing wrong. Would appreciate if you guys can help me out.
 
Thanks,
DJ DurhamDesi