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2000 Chevy, 2500, 5.7 It sounds like a broken belt coming from the dash. It does not stay in time with engine rpm's. It comes and goes at any speed from idle to highway speeds. It does not make any difference if the HVAC system is on or off. Just replaced the air blend actuator and found heater core has been leaking which will be replaced next week. Does it make sense to anyone that the leak could somehow be connected to this noise? Chime in please, and thanks.
 

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Yea, Im thinking its in the hvac box somewhere. A stethoscope may be helpful in finding the issue.
 

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2000 Silverado Z71 4x4 5.3L 460k+ miles w/ GM rebuilt motor and trans
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Could it be an actuator slipping? Our '04 Burb had one slipping intermittently that would make a popping noise as it jumped the tooth. This was not tied to the AC being on or off, as it would try to index when the vehicle started, give up, and try again later. Sometimes it would last a minute. Other times it lasted the entire drive.
 

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I have the same noise on my 05 sierra last a good couple minutes then goes away.

Summer time the a/c on the driver side will go to hot turn truck off and on back to cold.

My 2005 sierra does it had a 2000 Pontiac grand pri also did.

Not sure what to change since it was very sporadic.
 

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Discussion Starter #6
The noise comes and goes with fan on or off. With HVAC controls off and actuator laying in floor (disconnected) while I went to pick up the new actuator.......flap, flap, flap whether I"m driving or stopped.
 

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The blower fan is a "squirrel cage" design, there is probably something caught in the fan.
If like mine, it's behind the glove box and should drop straight out
 

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The noise comes and goes with fan on or off. With HVAC controls off and actuator laying in floor (disconnected) while I went to pick up the new actuator.......flap, flap, flap whether I"m driving or stopped.
The HVAC system in these trucks are nuts. I never really noticed how mine acts before I installed electric cooling fans. I tied the relay into the AC clutch switch so that the fans would come on when the AC compressor comes on, along with the temp switch, and a switch on the dash just as a back up. With nothing on ( heat, ac, deforst, ECT. ) the AC compressor comes on out of the blue. This turns on the fans, and thats why I notice the compressor kicking on. IDK if it does that as Im driving, but it happens all the time at idle. It doesnt last long, only a few seconds to a min., but nothing is on. Have you pulled your ac fan, and looked inside the HVAC box. Wont hurt to look inside the box. You can pull the filter while you have it open. Mine had fallen apart more or less. I just discarded it, and Im running without one. They only way to access the filter is to pull the fan.
 

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Here are a bunch of videos on pulling the fan
 
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