In safe mode windows wills start itself with a limited amount of files and drivers. Which can help determine if your problem may be driver related.
So first you should go to your motherboard manufacturers website for updated drivers. Including audio driver. If your not sure on the name and model of your motherboard you can download a nice little program called CPU-Z http://www.cpuid.com/softwares/cpu-z/versions-history.h...
Download the "no installation" one and your motherboard will be listed under "mainboard" within the CPU-Z program.
After that, It wouldn't hurt to make sure all attached devices have updated drivers as well.
Then I would start with hardware troubleshooting. Remove all USB attached peripherals. Keep only the keyboard plugged in. Boot, Log in, see if the sound in question happens. Then plug in the mouse and boot, log in, see if sound happens. Continue until you find out either (A.) what device it is causing that sound or possibly (B.) what port it is.
Let me know how that goes and we'll go from that point.