27" iMac 3.5mm audio output problem

I'm thrilled with my new 27" iMac. Loving it but discovered a problem. I have a Bang & Olufsen Beoplay A8 speaker and I used 3.5mm male to RCA cable to bypass iMac's speaker and use A8 instead. The problem is when I turn the volume all the way up on my iMac, there is this white-noise like sound affecting the sound quality. However, if I don't turn the volume all the way up on my iMac, it seems to work fine regardless how loud I turn up my Beoplay. I tried the cable with my iPhone (volume all the way up on iPhone) and it was fine. So I suspect it has something to do with iMac's internal. Can anyone who have the new iMacs test it out? Or any input would be helpful. Thanks!