My truck makes loud click and engine slows up and down whenever I turn heater on. Any ideas why?

 

Welcome to the forum

If your Ranger has Air Conditioning then the AC compressor needs to come on when the Climate control is on, especially in Defrost mode, AC is used to DRY the air in the cab to defrost window quicker, it doesn't "cool" the air in this mode it DRYS the air
And AC is also used in general to DRY the air in the cab with any setting except OFF and VENT, but depends on the YEAR of Ranger

And compressor can cause changes to RPMs, the "click" may be the Clutch on the compressor turning compressor on and off, and since its not being used to "cool" the air it often cycles on and off quicker than when using AC for cooling