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Chirping Sentra

I recently purchased a 1999 Nissan Sentra with 189K on it. The car runs great and the AC ran ice cold. The porblem was when the AC turns on a very loud noise is heard it sounds like a million crickets chirping.
I took it to a mechanic and he said it was a bad AC Compressor. So I replaced the AC compressor, belt tensioner pulley and serpentine belt.
The noise is still there.
So here is my further troubleshooting:

  1. the noise is clearly present when the AC is on.
  2. With the AC off the noise is heard only in short burst of maybe a second or two.
  3. When I remove the FAN relays (all 3) the noise goes away (with or without AC on).
  4. If i leave the fan relays in a disconnect the FANS power, you can still hear the noise. (So the fans are not the issue)
  5. I also removed the AC clutch and the noise is still heard.

HEEEELLLLPPPP!!!

Looking at the wiring, the fan relays just control the motors.
The motors seem to have three speeds (on an automatic anyway). Taking the relays out or disabling the fans should have the same effect – unless the ECM (it actually drives the relay coil) has a way to see if the relay has coil opened up, which is what it would see if you take the relay out. Maybe it decides you can’t have AC with the fan relays not in circuit so your noise is gone.

Have you listened around with an automotive stethoscope?

Yes, still can’t pin point the noise. It doesn’t seem to come directly from the Compressor but somewhere nearby.