I don't have grey tree frogs, but Music and the sound of water gets both of my green tree frogs going... especially the Six Flags song "We Like to Party" and the Gummi Bear song on YouTube. Also when dumper (WTF) calls, they respond to him, but he doesn't respond back to them, he just likes to talk to himself.

It's crazy how they sound like little dogs. I always refer them as "barking" but the other night one of them was doing that beepy "quacking" sound that I've heard on all the audio clips on the internet. The two calls must have different meanings.