Why do we need hard steel balls to produce an unusual ‘chirping’ sound? When two hard steel balls, or similar, are brought gently into contact with each other, 
an unusual ‘chirping’ sound may be produced.why do we need this material or shape to produce that sound?
 A: Whether this is what you're hearing I don't know, but I've often heard a chirp type noise from collections of steel and glass balls. This seems to happen when the balls slide over each other. My guess is that the sound is generated by stick slip friction as the surfaces slide over each other. An easy test for this would be to coat your balls in a thin film of oil and see if that affects the generation of the sound.
A: Chirp can be defined more formally as a signal that increases (or decreases) in frequency. See also this article. When birds chirp they produce sound that quickly raises in frequency and hence the name chirp.
My guess is that this sound is produced by the balls bouncing off each other. If you drop a small metal ball on top of another one they will bounce many times before they come to rest. Each time they bounce they rebound with less and less velocity. Less rebound velocity means it takes less time for the next bounce. At some point the time between bounces becomes so small the sound of the multiple bounces blend together to form one tone. Since the time between collisions decreases during this process it will produce a sound which increases in frequency and sounds like a chirp.
The reason this happens with metal balls is because they have a high coefficient of restitution. This means they lose very little energy during each collision. If you bounced two wood balls together they would bounce only a few times and come to rest.
Note this is only speculation on my part but I think this would be a plausible explanation.
