2006 Mustang GT Losing Freon and weird knocking sound

My car's still under warranty and I've taken it to the Ford dealer twice to fix this issue but it hasn't stayed fixed. My A/C keeps slowly losing freon. When it blew cold, there was a loud bang that would come through the firewall when I was at an idle almost everytime I'd come to a stop. I could feel it in the pedals. As it lost freon over time, the bang would go away. It seems realted to the compressor kicking on but I can't figure it out and neither can the dealer. They've replaced the schrader valve once and a condenser line the other time. I think they're just guessing. Has this happened to anyone else?