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hey guys on my ford f250 i have a intermittent knocking sound when i turn the ac/ heater on any idea? it is not all the time and can never tell when it is going to happen. i always think the worse like it is a cylinder or something any idea
A/C compressor clutch snapping closed. It will make a good thunk.
Also you may think the A/C is not on. But any time defrost is on
So is the A/C compressor.
Few times? I was reading tired when I replied last night.
Could be that it's getting ready to go if it's not holding
or also could be some other thing. Have someone start the truck
for you and stand at the passenger front corner with hood up.
The compressor is on that side and from underneath you can see
it. When the truck is started see if you can hear it in that location.
Then we will have a better idea on where it may be coming from.
Are you sure as to where it's coming from?
The A/C blend door can also make a thump.
Turn the A/C off start and see if the sound is there
or not. Then turn the truck off and let it sit for a 2 or 3
minutes. Try starting again. Any sound and from where?
If no sound then while it's running switch the A/C on.
Did you get ay thump?
Ok I've tried starting it without the a/c and it didn't do it. Started with the ac and it didn't do it it's really weird and it doesn't happen all the time.
If the sound is coming from the dash area.....it may be the vent damper door closing/opening. Check the vacuum pump located on the passenger side of the engine compartment next to the a/c blower. Check it for loose electrical connection and leaking vacuum hoses. My truck started doing this a while back after getting it back from the dealership. Found the electrical plug was not pushed in all the way. Plugged it in correctly and have not had any more issues.
