"knocking" from the freezer GE Profile...

Question

Our refrigerator had started making "knocking" sounds around October/November. It mostly happened in the evening around the same time so we thought perhaps it was a cyclical thing. Lately it's been occurring more frequently and much louder than before. The sound usually lasts for less than a minute. It literally sounds like someone is banging on a pipe inside the freezer section. Any thoughts? We've had this fridge since May 2007 so it's not very old. We've had repair men come to look at it but we're not really satisfied with their answers.