1999 Chevy silverado A/C Issues

Hi: Dave641 asking for help, my 1999 Silverado Ext cab 5.3L A/C is blowing very cold air from the rear vents, but less cold at the four front vents, the A/C was rebuilt completely, correct amount of Freon and oil, new BCM, new Engine, New Trans, New ECU, all four actuators replaced, preformed the so called reset procedure many times but still hear a clicking sound under the dash, hard to tell where it’s coming from since it only happens when the vehicle is moving with the A/C on, also there is very little water coming from the drain tube, yes I checked to see if the drain tube was blocked and as of Saturday 06/16/2018 there is no blockage, zero moisture inside the cab, what can be causing the air to blow real cold in the back and hardly no cold air up front, any suggestions are welcome, yes I have had it at my local dealer and as always they could find no problems?