Whistling or screaming sound when an old package unit is running.

What causes a whistling or screaming sound when an old package unit is running. Sounds like it’s coming from inside the evaporator coil and/or the compressor area. The first time I heard this was when the unit was shutting down. I’ve heard this kind of noise before when an old pkg unit was shutting down, and it made me think it was something like refrigerant moving… bleeding off pressure? First time I heard it on this job, it seemed abnormally louder then I remember hearing it once or twice on previous jobs.

On the same job after the unit was running for a while the noise repeated while it was running (not after turning it OFF), and it was extremely loud this time... I shut it down and left it that way. Told the owner not to run it and I would get help/advice and get back.

Just googled about the this noise and the first result to come up was about it being dangerously high pressure at the compressor, but it was regarding an automotive air-conditioning system.

Didn‘t connect gauges yet, and beginning to wonder if I should just stay away from it.

I have a bit more to tell about the condenser fan not running for who knows how long, but started running after replacing the dual run cap that tested well below tolerance for the FAN.
When first walking up to this unit, fan wasn’t moving and was making a strange noise in addition to typical humming, and I’m beginning to wonder if that strange noise wasn’t coming from the compressor. I could move the fan blade only a little with a screwdriver, but it didn’t bounce back like I’ve seen before when a condenser fan motor would lock up when the system was energized, and was told this locking up while energized was due to winding insulation having worn off and setting up opposing electromagnetic fields... power off and fan moves again normally.

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