I dunno, sometimes when I play 2 notes on higher strings, I can hear a very low note being played.
With amp overdrive and fairly loud, in Drop D tuning
G string, 7th fret
B string, 8th fret
After about 1 second you can hear a very low G note that is 2 octaves below the one played on the B string (1 octave below the open G string).
Essentially it sounds like you're also playing the low D string on the 5th fret. Makes for a HUGE bassy powerchord sound.
I figured it was just sympathetic vibration but I played the same thing while muting all other strings and you can still hear that lower G note.
I call it the brown note