jeep AC experts please!!!

02 grand cherokee, ac was low on 134 so i added some,now there is a hammering sound like a machine gun going off!!,even when you shut the car off it continues for a little while,it actually shakes the low side line when its running,i did a kinda quick search and it would seem that people who have had this problem replaced the expansion valve and it cured the problem,i would like some comments from a AC expert,there has to be some on here!!,the fact that it continues after the car is shut off for a while makes me think its NOT the compressor,and yes it will do it when the freon is a little low as well as high,i just want to be sure im not on a witch hunt when i tear into it!!
thanks in advance!!
Butch

Are you sure you didn't over fill the system?

overfilled a/c system.

ITS NOT OVER FILLED!!,its actually low!!when the low side gauge goes to 20 or so the compressor kicks off ,then when it rtaises to 30 or more compressor kicks in again,!!! to me thats a classic sign of low freon!!

could be unbalanced high to low, but I had a Chrysler 300 that hammered and it was a blocked expansion valve.

Are sure it isn't the hi pressure blow off valve. Are the radiator fans working. What is your hi side readings?

i evaced the system,refilled,no more knocking,but still not getting cool air,now have the blend doors out and going for a new blower motor,return sweats clear to the compressor,and low drops below 30 and compressor cycles?????????????

What are your hi side reading?? Better yet what are your low side and hi side reading at the same time?

replaced the blend doors tho they wernt broke!,also the actuator motor and fan motor,now getting really cold air during a low 70's day,high about 150 low 35 down to 20's when compressor cycles,a hot day will tell the story!!