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

Tiny
BENLEE1199
  • MEMBER
  • 2006 DODGE RAM
  • 5.7L
  • V8
  • 2WD
  • AUTOMATIC
  • 85,000 MILES
About two weeks ago my truck started making a loud knock or bump/thud sound about fifteen to thirty seconds after turning on the A/C. It sounds like it is coming from the floor underneath the dash, pretty much right in front of your feet, and sometimes you can even feel the knock.

It only does it one time, just one single knock, and it does not do it again for the full duration of the drive. It also does not do it every single time, but maybe about 50% of the time or maybe slightly less.

I uploaded a video so you can hear the knock, which is at the end of the video. It pretty much sounds and feels like someone is under the truck and hitting the underside of the body with a dead-blow mallet, which is why I would call it a thud as much as a knock. It is not metallic sounding.

Thanks for your input!
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Thursday, May 3rd, 2018 AT 6:12 AM

13 Replies

Tiny
PATENTED_REPAIR_PRO
  • MECHANIC
  • 1,853 POSTS
What is that knob you turned, was that for the blower fan speed?
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Thursday, May 3rd, 2018 AT 6:53 AM
Tiny
BENLEE1199
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  • 67 POSTS
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.
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Thursday, May 3rd, 2018 AT 7:34 AM
Tiny
PATENTED_REPAIR_PRO
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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.
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Thursday, May 3rd, 2018 AT 7:58 AM
Tiny
BENLEE1199
  • MEMBER
  • 67 POSTS
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.
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Thursday, May 3rd, 2018 AT 8:43 AM
Tiny
PATENTED_REPAIR_PRO
  • MECHANIC
  • 1,853 POSTS
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.
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Thursday, May 3rd, 2018 AT 9:00 AM
Tiny
BENLEE1199
  • MEMBER
  • 67 POSTS
Okay, I will try disconnecting the fan first and then go from there. Thanks!
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Thursday, May 3rd, 2018 AT 10:03 AM
Tiny
PATENTED_REPAIR_PRO
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You are welcome and post back if you need any more help.
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Thursday, May 3rd, 2018 AT 11:31 AM
Tiny
BENLEE1199
  • MEMBER
  • 67 POSTS
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?
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Wednesday, May 9th, 2018 AT 7:20 AM
Tiny
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  • MECHANIC
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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.
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Wednesday, May 9th, 2018 AT 7:29 AM
Tiny
BENLEE1199
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Okay, I will do that. Thanks for the diagrams!
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Wednesday, May 9th, 2018 AT 7:38 AM
Tiny
PATENTED_REPAIR_PRO
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You are welcome.
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Wednesday, May 9th, 2018 AT 8:09 AM
Tiny
BENLEE1199
  • MEMBER
  • 67 POSTS
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.
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Tuesday, July 13th, 2021 AT 5:45 AM
Tiny
DANNY L
  • MECHANIC
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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.
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Tuesday, July 13th, 2021 AT 10:13 PM

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