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When I start my car, and while it's still at idle and stationary, I hear a noise which sounds like a fan or belt hitting something, but at a low frequency of 4 Hz. The sound is like rubber or plastic impacting something and is not a chirp or squeal like a belt. After around 20 seconds, it stops on its own.
I've narrowed it down to the "Inside Air" function. When I switch it on to recirculate inside air, it makes the noise for several seconds. It happens every time I switch it off and on again.
This kind of problem can be tough to sort out. Let me offer a few suggestions to get started on tracking down the source:
Get a helper to work with you. Have her/him start the car while you try to localize the source. Having the hood open will help. Walk around as you listen to the sound. You can use a long screw driver as a "stethoscope" to help you isolate the source of the sound. Hold the blade against the suspected source and the handle to your ear. If the blade is near the sound it will be very clear and evident.
4 Hz is about 240 RPM, that's low for most things on a car - your idle RPM is around 1000 RPM and anything driven off of the belt will be faster than that. Most of the electric motors will also be in the 1000+ RPM range. This suggests to me that the sound may be coming from a solenoid or other control actuator rather than from a spinning component.
With the engine off, look closely at the belt(s), things driven by the belt(s) will usually be running faster than 240 RPM, but the belt itself will be making "full transits" at a slower speed, so perhaps you're hearing the belt hit something. Look for cuts or loose sections of the belt. With the engine running does the sound seem to emanate from the area of the belt?
Vary the engine speed and see if the noise varies with it. Sounds caused by anything driven by the engine will increase in frequency.
Turn the key on, but don't start the engine, do you still hear the noise? If you do, do you hear anything else at around 20 seconds (when the sound stops)?
Use the information you collect to clarify your question. If you can add photos of the suspect objects and/or video or audio of the sound.
One other thing to consider is that this sound is completely normal and that something else has changed to make it more evident. For example if it is related to the A/C perhaps the cabin air filter was removed and is no longer muffling the sound.
You should tell us the brand/model of your car. If you have manual or automatic air conditioning, if you have AC compressor etc. Because different cars have different air conditioning systems and it would help tremendously to know exactly what yours look like.
Well my immediate guesses are:
1- The air conditioner uses flaps to divert air. If your ac is using servo motors, they may be broken so when you switch to inside air, it may be causing servo to skip teeth (which will make plastic clicking sound) while trying to operate the flap. Usually you would hear a smooth motor sound first moving the flap then it would click few times until it gives up. Sometimes it is the variable resistor inside the servo is broken, system may move it back and forth trying to set correct resistance.
2- If you have AC compressor, this will turn on when you switch to inside air. (even on cold days to remove humidity from inside air) and for some reason the solenoid may be engaging/disengaging.
If it's related to the Inside Air function, the first place I'd look would be the ventilation ducts, esp. the inside air intake. There might be a leaf or something like that stuck in the system.
I suggest it's a squirrel cage fan with bad bearings or a broken blade. Once it reaches speed, it tends to balance out.
I would check the condition of all the inside AC related blower motors and fans.
I have the exact same issue. I suspect it's related to either the belt or the pulley. Check if they're aligned properly, and there is excessive wear.
