I have a pair of monitor 60s that are a little over a year old hooked up to a Denon 591. Recently one of the 60s started making a crackling noise occasionally. It happens during movies, games or music - played through multiple different sources. It is not a constant crackle but happens every now and then. It seems to happen more often when higher frequency sounds are playing, but occasionally happens during regular scenes/sounds. I tried switching the speaker placement to see if it was the receiver channel out but the problem followed the speaker. Any suggestions?