I having a sporadic knocking from the bottom of the reffer side

After about 4 years I have had a knocking from the bottom, This is not the ice maker! the repairman has been out 3 times to "try" and fix the problem. Just like your car the knocking NEVER  occurs when the repairman is here.  The knocking seems to occur mostly in the evening, each time it only last for about 20 40 seconds.  It strong enough to be felt when sitting in an easy chair about 6-9 feet away.  The last time the repairman "bent" some brass tubing behind the cardboard cover,,, telling me it was something to o with the new type of freon now being used.  this seemed to make a small difference, it did not knock for about 10 day. Now it has started back again, when it started again it was not as strong or as long in duration. But it was started ti increase in strength and duration,,,, so anybody got any solutions ???