[Elecraft] K2: what is this strange intermittent buzzing sound?

[Bob]

My K2 seems to have a rather strange intermittent buzzing sound that occurs
only when I press DISPLAY for the display to turn on/off the DSP noise
reduction and notch filter. The buzzing sound goes on and off at an
approximately 1 sec cycle.  This occurs only when the NR and notch filter
settings are displayed and it goes away when DISPLAY is pressed again to
display the power supply voltage. It doesn't seem to occur under any other
circumstances. It's more noticeable when there is no antenna attached and
much more noticeable when in USB/LSB rather than CW (probably an affect to
the wider bandwidth when in USB/LSB).  Turning on the DSP noise reduction
reduces the volume of the buzzing sound along with the background noise.
Other than this intermittent buzzing sound, everything appears to be working
normally. I don't know if this is a new thing or if it's been there all
along and I never noticed it. So, what is this?
 
Thanks.
 
Bob W1SRB