[Elecraft] Birdies

[Wilson Lamb]

I've meant  for years to ask about the birdies I hear from my K3. 
At many frequencies across HF, I hear obviously internally generated birdies. 
They are very narrow and quite strong. 
I've noticed recently that I can cause them to occur by setting the high and low ends of the filter to overlap (zero or negative bandwidth), but there are still come heard with normal settings. 
I see this regularly, on two K3, so feel it must be something experts know about? 
Thanks, 
WL