Cpu fan rpm issue is probably due to old cmos battery so first try replace cmos battery with a new battery.  After that use motherboard CLR cmos jumper to clear cmos settings.

The motherboard password jumper is located next to the lowest PCIex1 slot.  Follow instructions printed on motherboard.

I was able to remove the BIOS password and in turn enable AHCI mode.

Well the fan now spins a little faster, however, not like it used to do before.

I will try to do what you say about the battery because as I say this originated from updating the Bios, and while trying to remove the password I removed it for a few minutes and when I put it back on it made 5 beeps and turned on the machine, the beeps were not heard again, but I have not had problems with the time or anything like that

Update: Well I ran the Dell diagnostic test and it gave me these results and as you can see the fan RPM is very low, and I remember it should be like 5000+ at max power.

 

 

 

Also while doing it, it came up error along with about 5 beeps constantly and checking I saw that it is the BIOS battery.

So I should change the battery?