Whirlpool Refrigerator

Question

I have a 3 year old Whirlpool refer model ET1TTXKQ00, that makes a loud drumming noise from the lower, left, rear side. It lasts 2 to 3 seconds and is on an erratic schedule. It dosen't sound like a fan rubbing, but I havn't pulled it out to check it. Any help is appreciated.