What is it and what's that noise?

The noise is the sound Monkey's Geiger-Müller counter emits to signal that alpha particles are ionizing the Argon gas within it's detector tube.

Or as Monkey says, "The stuff coming out of the glass clicks the stuff in the tube and the machine lets us listen."

As for the Vaseline Glass, it contains uranium dioxide. The amount is enough to make the glass green or yellow-green in color (especially under a black light) while remaining safe to own and enjoy. Notice how the meter rates the glass chicken as only emitting about 3 milli-Roentgens per hour. That was a peak reading. Most of the time it was much lower during Opice's test.

Monkey collects uranium glass because it originally came from his homelands and he owns one small saltcellar made of it.

"And now that I have one I don't need to collect any more 'cause I'm saving room to collect other stuff, too. Like gold and diamonds."