This is currently happening to 2 Win7 PCs where I work. Whenever the computer is locked the device plugged in and device unplugged noises play continuously. The users logged in when the noise happens are not local administrators, and it appears not to happen when a local admin is logged in or the PC is in safe mode. There are many other machines with the same hardware and same software image on the same domain that aren't having this issue. Any ideas would be great!
This answer is very late, I had forgotten I posted here but was looking through my history and saw it.
tl;dr manually set the monitors input rather than using auto-detent
The monitors we were using (Dell's, not sure of the model) were set to "auto-select" their input. When the PC had been idle for long enough to stop displaying on the monitor at all (after screensaver) the monitor would start flicking between all of the available inputs. When the monitor changed from the input the PC was using, the PC would detect this as a device being disconnected, when they switched back to the right input method, the device would be connected. This would continue until someone woke the PC. To fix it, change the input from auto-detect, to the input you are using.