Taz, If your fridge is installed in a slideout I believe it is a fan you're hearing. I know when we first got ours I heard something different also and after checking the ice maker and taking the outside lower vent off I discovered the fans. from what I understand with the upper vent not on the roof the venting isn't that good unless they install the fans to help push the air out the top side vent, it just doesn't draw right when they can't use the normal fridge vent on the roof.