Windows keeps making these random "device connect" and "device disconnect" sounds

Problem

Throughout the day I notice that my computer makes apparently random device-connect and/or device-disconnect ("boink") sounds. I suppose it is the same sound you hear when connecting or disconnecting a USB device such as a thumb drive. I've noticed that this happens on each of three computers I work with at home, my wife's computer, and my machine at work. It happens without any user action at all - i.e. I'll be just sitting there (hands off my mouse and keyboard), and the computer will make the sound. There is no visual queue or anything. Just the sound. I have sometimes gone in pursuit of the sound - running virus scans, examining event logs and such, and observing task manager - but have never had any luck tracking this thing down, but have not had any luck. I've been noticing this since Windows Vista and now the sounds have followed me through to Windows 7. Surely someone else out there must be experiencing this and can explain what is happening.