Loud (single) knock sound when turning on A/C

What is that knob you turned, was that for the blower fan speed?

Yes, that knob was the fan speed. It was off and I turned it to "1". The A/C button was already engaged previously, so when I turned on the fan speed the A/C came on automatically.

Okay, the AC was already on, that turns on the blower fan and since that noise appears to be coming from that area I suspect it has something to do with the fan. To make sure, unplug the blower motor wire and listen if that noise goes away when moving that knob from off to on. If so, check that the blower fan squirrel cage nut/bolt is tight, so you might have at actually remove the blower motor.

Okay, I will give that a try. Although it sounds like it is coming from under the vehicle, not from inside under the dash. Also, to my knowledge, the A/C is not technically "on" or engaged until the blower is turned on, because the A/C light does not come on until the fan is turned on, and that is when you can hear the A/C compressor click on like normal, and the knock comes fifteen to thirty seconds later.

If the noise does not go away after disconnecting the blower motor wire, and you are sure the noise is coming from under the vehicle perhaps you should either jack it up and place or jack stands or up on a lift have get under there and listen while someone else turns the fan off and on and try to pinpoint what is making the noise.

Okay, I will try disconnecting the fan first and then go from there. Thanks!

You are welcome and post back if you need any more help.

I have been so busy that I have not had the chance to try disconnecting the blower fan, which I will still do, but I noticed something else very strange when sitting in my truck yesterday waiting to pick up my son from school. When the AC is on and with the fan on high, the air coming out of the right two passenger vents is noticeably warmer. I kept putting my hand right in front of the two driver side vents and then in front of the two passenger vents, back and forth, just to see if I was imagining things, but the two driver side vents are definitely colder than the two passenger vents. The passenger vents almost felt like they were just blowing warm air with almost no cooling at all. Do you know what could be causing that, and could that be related to the knocking sound when turning on the AC?

It could be related and it could be a door actuator not working and making that noise.
You would have to gain access to the actuator shown here as #3 in this first diagram and see if it works when you move the AC controls.

Okay, I will do that. Thanks for the diagrams!

You are welcome.

Just for the benefit of others, I wanted to mention that simply charging the A/C system with refrigerant fixed the problem completely. No more knock from the A/C in the last 2 years.

Hello, I'm Danny.

Thanks for sharing. It can help people with the same car and issue when you provide updates. Thanks again for using 2CarPros and we hope you will use our site again in the future when needed.