2000 Nissan Altima whistling noise when the engine warms up

If the defroster is turned on, the air conditioning compressor will be cycling on and off to remove moisture from the air before it blows onto the windshield. On most cars, the AC condensor fan turns on when the compressor does. If you have two separate fans, double-check if it's the radiator fan or condensor fan. Some cars only have one fan for both systems.

There are a number of dandy clues you provided, but there are a few different things that could be making the noise. Can you find a mechanic's stethoscope to poke around by the alternator and the AC compressor? You could also remove the alternator and AC belt(s) to see if the noise still occurs.

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Yup, I did. Have you listened with a stethoscope? Fans turning on and engine speed dropping suggests the air conditioning is running as it will when the heater is in defrost mode. You could be hearing a whistle as the refrigerant flows through the expansion valve or the receiver drier. There are mufflers in the lines that are supposed to quiet whistles and the pulses of pressure from the compressor. On most cars, the compressor cycles on and off to build high side pressure and to save fuel mileage. Even after the compressor turns off, refrigerant will continue to flow as determined by the temperature sensing bulb in the dash. The flow can turn on and off independent of the compressor. If this is what is happening, the noise will stop shortly after switching to the heater instead of the defroster. You might be able to feel the vibration too if you grab the high side AC hose.

You can also remove the belts and run the engne for short periods of time. If the noise still occurs, you can eliminate the power steeing pump and hoses and the alternator. Neither of those should cycle on and off like you described though.

If the AC hose seems to be the source of the noise, look for signs of a missing two-piece cast iron weight that was strapped around it, or loose or missing anchors. Most hoses are attached to the body sheet metal before they pass through the firewall.

You might also look for an emissions system purge hose coming from the charcoal canister. A solenoid opens to draw out stored fuel fumes to be burned in the engine, but to my knowledge they do not turn on except at highway speed.

Since we're looking for something unusual, listen with the stethoscope around the water pump too. A rubber lip seal can cause a squealing noise although it is more common for axle seals to do so. Perhaps it's related to the amount of pressure in the cooling system. The pressure will go down after the fan turns on and coolant temperature goes down. Proof of that could be found by running the engine with the radiator cap loosened one notch.

You can buy a stethoscope at the auto parts stores for around 10 bucks. They are real effective in helping to find these types of noises. Also consider a squealing camshaft seal, crankshaft seal, and timing belt idler pulley bearings although you would think they would make a noise constantly.

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