APC SMV1500 AI-MS FAN ISSUE

[Maximus]

Hi, Im glad finally I can post here.
last week I bought an APC UPS SMV1500. It works well, but noticed a weird behaviour with its fan on the back.
Fan starts at Full Speed after I plug in to power grid, then gradually lowers speed. After a minute or so it stop, but fan starts to screech quite annoying.
By experience I know thats caused because power is too low to fan keep spinning. So I tried with another fan, I had an old CPU which used for test. surprisingly the CPU fan kept spinning at low speed.
Then I tried a Noctua NF A6x25 FLX (60mm) as replacement...sadly it fails at lowest speed, and it never recovered until I restart the unit (strange) So I cant get a decent replacement for this fan.

My doubt is if its how its intended to work? Fan Stopped but screeching like that? If I get the exact same replacement for the fan Im afraid it will stop as well.
I cant even make it with fan disconnected if I want to install a custom controller, because fan error alerts.

[Alm77ar]

Same issue with same model