Strange metronome-like sound when recording

hello,

when i record on an audio track using the built-in microphone i often hear a metronome-like synth bass sound when no such track is "solo'd". What i am saying is that i can only have the audio track "solo'd" (you know, the yellow 'S' button) while recording using the built in microphone and i hear a pulsating metronome-like synth bass sound that repeats as i am recording.

what could be going on?