What is that repeating sound throughout the "Moose in the Morning" episode of "Sunrise Earth"?

This episode features a mother and baby moose feeding in a pond at Baxter State Park in Maine, USA. Throughout there is a dominant repeated sound -- a sort of clacking or tapping is how it seems -- that might be a woodpecker tapping or a beaver munching a log or a bird clucking near the microphone, or maybe it's the mother moose making some some sort of "here I am" sound for the baby.

Can anyone identify this sound, please.

Curiosity is killing my cat, you see.

Thanks very much.

Being from Maine, I can identify that sound, I believe what you are referring to is a frog, or frogs croaking, the sound may vary some from frog to frog species, but the clicking sound, limp guitar string, croaking, or groan, all types of frogs trying to find a female who finds the sound alluring.