That's the highest tuned bass I have ever heard...
I don't think it's the speakers, but it's really hard to tell just by listening. It almost sounded like a snake drum buzzing! Sounds like a vibration/buzz, but it's really high. What does it sound like when you play a low note through it? Does that sound come out of every speaker? Regardless, I would pull the speakers out of the cab and play a little music through them at low volume and see if they sound clean, one at a time. Bypass the crossover. Could it be the crossover in the cab? Try bypassing that. Also, check the horn. unhook it to see if the sound goes away. I personally would just pull each driver and make sure each is good, then test the crossover, then rewire and make sure it's right, then look for cabinet vibrations, etc. Maybe have a friend try to isolate the noise while you play.
edit- Im almost positive that is what is coming out of your "guitar" amp. I would check that again...
Wes