barking cat

Hi, guys. I'm trying to build a little gismo for conditioning reaction times. Baiscally, all it has to do is beep at random intervals. It sounded simple enough when I first thought of it, but then I realised I have no idea how to make something vary randomly within a specific continuous range. I've searched the internet, but can't find anything that looks remotely relevant. Can anybody here point me in the right direction? Keep in mind that I am a relative newbie, so I am after the simplest possible solution. Thanks