I went on a call where a customer was having compressor noise issues with his York 18 SEER heat pump in cooling mode at low outdoor temperatures. I haven't had much experience with cooling at low ambient, I seemed to be able to reduced the noise a bit by raising the head pressure. If low temps are indeed the cause of the noise, is there an easy way to boost head pressure? I tried to install a low ambient kit a while back to slow fan speed but was either too stupid to make it work, or the equipment might not have been compatible.