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It's normal for there to be some noise as the pressures equalizes across the orifice tube after shutdown. However, it the AC doesn't work tomorrow, you know it has a leak.
Also not knowing how much refrigerant is in the system and then just
dumping some in is not a good plan. The right way is to measure pressures
or the better way is evac the system and put this right charge in and
maybe add some dye and have a look to see if you can find any leaks
with the UV lamp.