[Elecraft] [K2] NB2 heard a "spark gap" like signal 8k lower than the RX frequency

[Wayne Conrad]

On my K2, in CW mode, and when I crank up the noise blanker by selecting 
NB2 (and the default noise-blanker threshold, "Hi"), I sometimes hear a 
strong signal some distance away from my receive frequency.  The signal 
I hear, when this happens, is not a tone.  It's more like a spark-gap 
buzz.  Today, when the receive frequency was 7.034, I heard a CQ on 
7.026.  The signal on 7.026 was quite strong but otherwise sounded 
normal.  FL1 (1.50) was selected.  AF2 is installed but was off.  If I 
turned the noise blanker off, or select NB1, the interference goes 
away.  It doesn't happen often, and is seldom strong enough to be a 
problem, but it does make me wonder if something is out of whack, or if 
I'm operating the radio improperly.  Is it just the nature of the 
noise-blanker?

73, Wayne Conrad KF7QGA