I recently got an iMac (27" 2010 model) and when I have my Rokit 6 KRK monitors plugged in via RCA to 1/8" adapter, I am getting a low buzzing sound. The sound gets louder whenever I scroll the mouse or the hard drive starts spinning. Any idea of what could be causing this?
Plugging active monitors (or basically anything sensitive if you had a good ears) directly into onboard audio is almost never a good idea. There is little exception to this rule. To get clear audio output, the best way is to invest in a professional audio interface, for example, the el cheapo '0202 USB' from creative labs (eh, their subsidary, E-MU).