AC Relay clicks when in the vent position

The car is a 1986 S15 Jimmy 4X4 with the 2.8 engine.
Recently I noticed a loud ticking sound. At first I thought it was a spark plug wire off. But the sound is 3 1/2 clicks per second no matter what I do. I got a stethoscope and traced the sound to the AC Compressor relay. The sound is only there when the air conditioner control is in the vent position and no matter what speed the blower is set on. I immediately checked the heater position and it does not click then. It is only present when the engine is running.

I checked engine speed and it is a little higher in the vent position, maybe 100 rpm more than it is in the heater position or in the off position.

I added a can of coolant to the AC with no change.

I can't figure where to test next. Any ideas would be appreciated.

Jim

try switching the relay out with another one?

I don't think there is a rationale to worry about the relay. The AC relay is doing what it is supposed to do, closing when the Engine Control Computer tells it to close. The question is why is the ECM telling the AC relay to close when the AC control is not in an AC position. And why is the ECM telling the AC relay to close 3.5 times per second when the control is in the vent position.