Hi there,
anyone hearing the so called third sound (also called difference tone or Tartini tone, from the violinist who first described it in 18th century) while playing bichords on Harmonica (two notes together)? On a C-harmonica, for example, while blowing 78 I can hear a deeper tone. Try to alternate 78 and 89 and the pitch of this deeper tone will change.
The third sound is typical for double-stops on stringed instruments (double stop = two strings played at the same time) and its pitch is the difference in frequency of the two notes you play (so always in the lower register).
Cool thing to have on Harmonica, too… extra bass when playing in the high register 
At least is always in tune 
