I had a trojan a few months back that started a program that sounded like a radio broadcast every so often for what appeared to be no reason or particular pattern.
If you open your task manager up when the noise appears, look in the processes part and see if theres something running thats just a letter then .exe
I had various ones coming up from a.exe to g.exe.
If you highlight it and click on End Process the sound should go.
If it t day, look in C/Temp folder and on mine I had all those a.exe and b.exe files in there. I deleted them all and after scanning with a trojan remover, malwarebytes, AVG and every other clean up program that I had it got rid of everything.