99 Ford Ranger,4.0 I hear a chirping noise,sounds like it`s coming from back of engine.Also,when gas pedal is pressed a/c stops blowing out of vents,only comes out of defrost vent.I have changed serpentine and tensioner.A/C motor appears to run fine,also recharged a/c unit. Also have a noise from intake that sounds like a train.

 

A/C output moving from vent to defrost vents when applying the throttle means that you have a vaccum leak in your A/C, Heat, Vent system. Check your vaccum cannister for a leak and all of your vaccum lines under the dash and under the hood that are associated with the Heat and A/C system.