I just did this job on an '05 V6. Replaced evaporator, expansion valve and desiccant element - all Denso sourced from Rock Auto. Used the Four Seasons complete system O-Ring and Seal kit and replaced all the seals where I took anything apart. Did not replace the seals that I didn't touch. Even though it was in good shape, I ended up replacing the condenser because I couldn't get the nylon plug out to replace the desiccant element. It was seized. Your whistling may be debris, missing gasket or bad door seal in the air box or the duct system. You will have it all apart so you can do a good visual inspection and clean it all up. Basically you have to remove all the dash components to the firewall to remove the air box which contains the evaporator. Not a difficult job, just tedious. Info on the job in the Maintenance sticky on this forum. I used the FSM which lays it out step by step but it does bounce you from section to section.