Super Scary Masks

How to Make a Loud, Scary Sound?
I recently got a job at a haunted house this year and despite having a super scary mask and a rockin' hiding spot (I jump out from behind a curtain that is practically impossible to see with the 3-D glasses on (yes, you would think being in the real world would make it 3-D already but the customers get these glasses regardless and they actually do seem to make things pop out a bit more)), I'm not getting the scares I want. Sometimes people will laugh or others will mock me and yell back at me. I read somewhere that if I could perhaps startle the people with a loud sound just before I jump out, I could have a better chance of really freaking them out. So, I was wondering what you all recommend. I don't have a lot of money to buy stuff with, I can't download apps or anything like that cuz my phone isn't that cool, I obviously can't bring ANY KIND of weapon (this includes pellet guns and so forth) in with me to work, and I would prefer for the sound making device to look threatening just in case I have to run after the people because for some reason I doubt people will be afraid of a killer clown running down a hallway after them with a wooden spoon and/or pot in her hand. So ideas? Please!

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