Here's Mojo's text:
"Q. Why does my metal mouthpiece chirp and squeak when I play it?
A. I have thought quite a bit on the physics of chirping. The driving force is high pressure. Players almost never chirp at low sound volumes. If a piece is uneven or the facing curve has irregularities, it will not be responsive and a player needs to use more pressure to make it speak. With high pressure often comes a tighter embouchure that closes off the tip opening some. This geometry is prone to squeaking. Especially if the tip is uneven and/or the baffle is high near the tip. It allows the very tip of the reed to vibrate like a mini reed at high frequency. The high baffle focuses the air even more. Try taking more mouthpiece in and/or playing while forming an "Ahhh" with you oral cavity.
The mouthpiece material has no effect on squeaks in my opinion. However, metal mouthpieces usually have higher baffles than non-metal mouthpieces. So it is common to think that metal is louder and squeaky. But this is not due to the material.
Players who like to play on the very tip of a mouthpiece are more prone to chirps. Players who take more mouthpiece in have less chirps. When you take in less mouthpiece, you usually close off some of the tip opening with your embouchure. A tight embouchure can do the same thing. Often you choose a harder reed to make this embouchure work for you. Or you start with too hard of a reed and this forces your embouchure to be tight or towards the tip. This is not necessarily a bad embouchure. Just be aware of the factors at play here.
Reeds that have a thin tip and a thick heart are more prone to squeaks, especially when used with long mouthpiece facings. A Fibracell reed has a cut that is difficult to make squeak. When I use a Fibracell and take in a little more mouthpiece, no mouthpiece squeaks for me. However, some players do not want to use Fibracells or change their embouchure. Then, working on the facing curve, tip rail and lowering the baffle some usually helps."