[Harp-L] Acoustic Feedback (newbie)

[Tony Renshaw <tonyrenshaw@xxxxxxxxxxxxxxx>]

I am wondering why I get feedback on my notebook speakers when simply 
playing my harmonica in front of them. I am practising throat vibrato on 
hole 2, and if I resonate a while, I can occasionally get the feedback 
happening. I thought feedback had to do with an amplified source, such as a 
microphone, not simply just another noise source??? In anticipation.
Reg
Tony Renshaw