You need to find the longest bit of forest tone without any other sound on it that you can, and loop it in multitrack so that it lasts the duration of your new audio. You may find that you need to overlap a few loops slightly to avoid the obvious sound of loop joins, but with a little experimenting this usually works better than you might think. If you can find several bits and link them the same way, it will be even better.
On another track, add your new audio. Go to the Mixer and adjust levels to taste. And when you play back both tracks, you'll end up with the sound of a closet in a forest. And that, I'm afraid, we can't help you with!