Sorry that I am a bit late with your message, as for the entry, I'm glad you find it interesting!
I'm hoping to make it easier so people can see the ending, I definitely made the experience a bit too hard!
Thanks for playing it, though!
To answer your question, there is only one monster.
I've had it so you also emit footsteps (because who doesn't like extra sound effects?), so it may or may not been yours.
As for the other high-frequency noise, I had forgotten to mention that those are walls, which should now be mentioned. Thanks for the feedback, though!