I drive a 2000 Infiniti I30t same engine as 2000 Maxima (3.0L) and the alternator were replaced by the dealer 1 month ago, but now it's making a weird pitch noise when the car is cold then as engine temperature rises the noise is almost eliminated but not at all, but, when you switch the A/C on the noise is gone completely, what can it be ? a bad remanufactured alternator ? what do you recommend me to do ?

 

I would check the tension of the accessory belt. I suspect that it may be too tight. When you select the A/C the effective belt tension may be changing (slightly slackens and thus eliminating the noise) at the alternator drive pulley due to the load on the belt as it now drives the A/C compressor which is adjacent to the alternator. (This will depend on the D of R.) Excessive belt tension can be hard on accessory bearings and can also cause a whining sound. A lot of mechs. overtighten drive belts in the theory that they will stretch and slacken off too much and cause a squeal. I failed diode in the alternator can cause a whining noise from the alternator, but if the alternator seems to be charging the battery under load then I doubt that problem. Have the belt tension checked first.