Is it a sound produced through the speakers or is it a sound produced from inside the PC?! If it is a sound produced by the speaker, then you could have issues with your audio drivers or your sound card. If it is a noise being produced from inside your case, it could be a hard drive, case fan, CPU fan, video card fan... etc. To troubleshoot the speaker issue, you can start by going into the Event Viewer in your administrative tools section and see if any events are being recorded at the time of the sounds. To troubleshoot hardware failure, open the PC's case up and try to locate the position of the sound. Good luck!!