What is making that clicking sound outside?

[Bill Bukovec]

I hear that a lot. Always thought it was a chipmunk.

[Stephen Tashiro]

It's like the sound you make by popping your tongue off the roof of your mouth.

Many types of birds make such a sound to "scold" dogs and cats that they perceive as dangerous.

[michael langman]

I have spent hours in the woods and listened to chipmunks and red squirrels go on tirelessly for hours at a time. Pesky little critters.