Freesound

A free, collaborative online library of user-uploaded sounds (738,114 at the time of checking) run from the Universitat Pompeu Fabra, searchable by tag, with downloads requiring a free account.

How to use it in horror

Search for field recordings such as creaking wood, wind through gaps or wet foley, filter to CC0 sounds for unrestricted use, and layer and pitch them down into ambiences and creature textures.