Frigidaire refrigerator knocking noise?

I have a Frigidaire refrigerator model FRS6HR5HS81 that I purchased new 3 years ago. Since day one, it makes a knocking noise when the compressor kicks on. I called a repairman out 3 times. It will not make the noise when he is there. It is very loud and wakes me up at night. Does anyone have an answer? Thank you.