popping sound from right speaker only of imac

I recently bought a (refurbished) September 2013 model imac 21.5". Looks great, but only one problem when I play any audio/ video, streamed or local, there come a clicking and popping sound from the right side only. Sounds like fireworks from a distance. No such noise through headphone. I tried all solutions suggested on different forums. No success. Can someone out there help, before I decide to take it to the apple store or return it?