During their travels through a dense wooded area the high pitched sound of a scream tears through the peacefulness of the forest. The intensity of the scream made it seem like it was very close. Traveling towards the scream the forest seems normally active and they find nothing. When they believe they are near where the scream originated from ,off in a different direction another scream rips through the woods. Following the scream quickly they find themselves again not being able to find anything. This could continue indefinately.
The sound of the scream is actually made by a bird the size of a large fist. It is mating season and that is how the bird tries to attract a mate. The bird is territorial so will generally stay in the same area so return visits here may have a repeat of the scream if they don’t find it on their first search.
A character with good wood lore or perhaps an animal lore background may soon understand what it is, never actually experiencing it himself till now so the experience was quite unexpected.