Continuous tone as soon as I open up a song - cannot make it go away

Question

I was watching another newbie (to Logic, not to studios) in our studio making up a beat.
He had about three tracks.
I wasn't watching too closely, although I was seated by his side.
Suddenly, a continuous tone came through the speakers.
Nothing we tried would stop this tone. We tried:
- Muting all three tracks
- double-clicking the right-hand midi information part of the transport window, which the manual says will guarantee silence as it sends a reset signal to all midi channels
- shutting down Logic and reopening it - the tone is played upon startup
- removing all unused audio tracks (it's on a menu option)
- removing all unused tracks (another menu option) thus leaving just the three used tracks.
The tone is still there. It only goes away when we shut down Logic (can't be certain if it goes away if we close the song, can't remember) and it always sounds on opening this particular song.
What's happening, what could we have done to cause it, and how to remove it?
This is more a technical question to try to understand Logic more - what we actually ended up doing was creating a new Logic song and start all over again, so the work's not affected at all.

Answer 1

1) Feedback loop caused by having an audio track in rec/ready and having signal from your board feeding back into the inputs that happen to be selected for that audio channel. That happened to me today. Doh!
2) A Logic "thang" that produces a whine with certain audio systems and at low buffer settings.
3) The Test Oscillator plug was installed (maybe previously) into an audio channel slot and got turned on somehow (was the tone in tune?)
4) If you sit too close to a really good programmer, the interaction of brainwaves... no, that's not it...    ;-)