I have a 1998 Chevrolet C/K full size pickup...just got the battery and terminals replaced...truck starts up just fine, but there is a clicking noise in the cab when the A/C is turned OFF...sound goes away as soon as A/C is turned back on...What is causing this

Did this just start after having the battery replaced? If so one possible cause, there are electric motors in the dash that control the doors for the hvac system, for hot to cool and to change air from floor to defrost ect. These electric motors "memorize" there positions from one direction to the opposite direction. They loose there memory once the battery has been disconnected. These motors have potentiometers in them so they know their position that they are in. The potentiometer goes bad and cannot tell which position they are in, and the electric motors are strong enough to strip the gears in them, the gears slipping past each other cause a "click, click, click" about one click per second. The reason it clicks when the a/c is off is because the motors are trying to recalibrate, but they can't. Don't worry, these are usually relatively cheap and usually easy to install. Obviously this is just a guess, but these are extremely common to go bad.