deebee

blueicedj – was that on a Bb trumpet? If so, your "B" would be sounding a concert "A" and getting sympathetic vibrations from the guitar's A string. The D string might be helping out as well. Try playing your (written) F#'s – do you get a similar effect?

You might also try playing into a piano with its sustain pedal down – certain notes will resonate. You could even get a pianist to hold certain keys down, as used by Berio in his trumpet Sequenza.