Every30 min, or so, my refrigerator starts this pounding noise.that sounds like someone banging on my back door. it continues for about 2 or 3 minutes. and then slowly disapates and stops. Then starts the process all over again. I had Sears come out. it didn't do it when he was here. so he took $65.00 and left

Hi

Noise Coming from Inside
In case of a self-defrosting refrigerator, the source of noise is generally from the inside of the refrigerator. Basically, a fan is used in this type of refrigerator for proper air circulation through both the compartments; the fridge and the freezer. The placement of fan depends on the structure of your refrigerator and where the freezer compartment is located.

If freezer section is on left side of your fridge, the fan is usually located on the topmost part of the back wall. If the freezer is at the bottom, the fan is on back wall in level with top edge of the freezer section. In case the freezer compartment is at the top, the fan is on the back wall, near the bottom or top or center of the freezer. So all you need to do for checking whether the fan is faulty and the cause of noise is to open the freezer and push in light/fan switches. If the noise becomes louder, the fan motor is faulty. If lubrication and repair of the motor doesn't help, you need to replace it.

If the source of noise is from inside the fridge or the bottom-front side or the food compartment, it's usually caused due to a damaged defrost timer. Just replace the defrost timer and check out.
Noise Coming from Bottom
If you can hear the noise coming from the bottom, check if the drain pan is rattling due to improper placement. Place it properly. If required, tape it in place to stop the noise. If this is not the cause, but you hear noise coming from the bottom, most commonly the problem source is actually at the back of the refrigerator.

Noise Coming from Backside
First of all, you need to open the back panel. Some refrigerators might not have this panel or just have a net type panel. Once you open the back panel, check the following to fix noisy refrigerator problems.

The problem may be caused due to a damaged noisy refrigerator compressor. A compressor is a single part, without any other moving part and is football sized. It is usually black in color and situated at the bottom with some tubes or wires attached to it. If this compressor is making noise, which will generally be loud enough, it needs to be repaired by a professional. The compressor might also be needed to be replaced.