Just a word of warning from someone who's had to use blanks for film and theater in the past:
blanks can kill. On the other hand, the invincible clown idea is hilarious if you can do it safely.
In terms of noises, use Audacity or any other free sound program to make very low-pitch, low-frequency noise. Sounds barely below the audible threshold make some people uncomfortable or nauseous.
This album contains some strange noise tracks and creepy children singing, if you need source material. What exactly is your roommate scared by, anyway?
If it is a photo-montage screensaver, make another folder identical to his montage source, but with a space removed in the folder name or another inconspicuous change. Add a number of strange or disturbing (spiders, etc.) images to this new folder. Make it a hidden folder and change display options to conceal hidden folders, and make the screensaver use this new directory.
At first there should only be one or two such images, so it comes up rarely or not at all. Slowly add more and more weird images, until by Halloween the screensaver is entirely messed up. If you can somehow sync the scary noises with these images, bonus points.
Some other ideas:
- Move, but do not actually delete, computer files. I mean, we're being scary, not malicious.
- Find a way to reverse-wire the volume control. Then change it back later.
- Conspicuously perform some kind of maintenance on your computer. When your roommate asks, say it has to do with getting rid of junk files that can cause odd behavior. Make sure it is something he cannot do to his computer. If you have an old CRT, degaussing is a great example. If he complains about the weird night noises, suggest this type of impossible-to-copy maintenance.
- Borrow a friend's pet rodent (hamster, gerbil, hairless cat). When the sounds occur, "find" the animal in the room in the middle of the night, and throw it out of the room. If you can arrange a safe way to toss it out the window or something odd, do so. Have there be no more noises that night. The next night, see how long your roommate searches for another such animal.
- Have a friend call your room phone during the night at random intervals. Make sure they hang up and do not speak into the phone. (This works best with no caller ID.) Then, eventually, have them stop for a night. Then start again. Then stop. The roommate will lie awake at night waiting for the phone to ring.
- Get a small vacuum cleaner. Whenever your roommate leaves the room to go somewhere else, casually begin vacuuming until they are out of earshot. Do not use the vacuum when they are actually in the room. Never explain why you do this.
- Buy some potatoes and dress them in doll clothes. Talk and play with them as if they are your friends. Specifically name and dress one like your roommate. One day, bake and eat the roommate potato.