Well, this has never happened to me before: I'm playing back MIDI and audio simultaneously and when I stop the playback a loud roaring sound occurs stopping only when I click on the icon to stop the audio engine. Sonar is up-to-date.
 
I'm playing back through VST instruments and outboard synths simultaneously. (Real Guitar, Roland XV3080, EMU Pro 2000, Addictive Drums, Addictive Keys, SI Electric Piano).
 
Any Suggestions?  Thanks!