PC fans ramp up and no display

[Eld3nmmt]

I got an HP PC in which I upgraded the gpu and play some games on it. Yesterday turned off the PC as the usual and on the next morning I tried to turn on the PC and the fans started to ramp up and there was no display on the monitor. I got this PC from someone, the person gave me the PC in its own box wrapped with plastic which probably means it was never used but the actual computer was launched in 2017 I think so I don't know how long it has been sitting without being used.

Here are the specs:

1. PC model: HP Z2 G4 tower workstation
2. CPU: Intel core i7 8700
3. GPU(upgraded): sapphire pulse radeon rx 6650xt
4. RAM: standard HP ram 2x8GBs (16gbs total)
5. Storage: 512gbs nvme m.2 Ssd and a 4tb hdd
6. PSU: standard 500W HP psu, label says 80+ gold
7. Mobo: not sure what the model is, might have to check on the official website with my computer's serial number.

Here are the things I tried when trying to fix the problem and no luck:

1. Reseated RAM. Removed both sticks and tried each one in every slot and didn't work. I tried to turn on the PC without ram in which gave me 3 long red beeps and 2 short white beeps (this is a standard HP code when there's a RAM failure) which means the PC is actually detecting the RAM correctly. Cleaned ram sticks and the slots too with an electronic cleaner spray.

2. Reset cmos using the dedicated button, removed the cmos and waited more than 5 minutes.

3. Removed the gpu and tested it with the old gpu, a Radeon pro wx 3100. For this one I tested it on the other pcie slot that was never used.

4. Disconnected all the psu cables from the motherboard and reconnected them.

5. Removed the hard drive (the system is installed on the Ssd).

6. Removed and reinstalled the CPU on the socket.
7. Tested the PC on 2 different monitors and a TV.

Did everything above and didn't work, the problem still persists. This PC just gives me headaches. It works fine for the most part but when I turn it off and then turn it on hours later there's always a problem. Last week it was giving me 3 long red beeps and 2 short white beeps (indicating memory failure), I Reseated the ram multiple times and did a cmos reset and didn't work, when I was preparing to go to a friend's house to test the ram sticks I decided to test it one more time and the PC turned on with the same ram sticks. Don't know what I did to solve it but oh well, it worked. There's also a green light on the mother board which, from my research, indicates that current is passing through the Board
Now the problem I'm having now is when press the power button every fan starts spinning, I can also feel the hdd spin but I have no display on the monitor. The fans start spinning normally and after a minute or two it starts ramping up a lot so I turn off the PC by holding power button or pulling the cord. When I disconnect all psu cables and reconnect them and turn on the PC the fans start spinning at a speed above normal and after the two minutes it slows down for 2 seconds and start ramping up at a faster speed. I have to turn on the computer without ram and then after turning off, insert the ram again so that the computer turns on at a normal speed but after the 2 minutes it starts ramping up again. The only fans that didn't spin was the gpu ones, tested both gpus. But the gpu fans actually did spin when I did the test with no ram installed so I think that may not be the problem. You guys got any ideas? It may be a motherboard issue but I've never had any problems when the PC was on.It's always me turning off the PC correctly by shutting down and then trying to turn it off hours later and not working.

Also the PC has a power supply labeled DPS-500AB-36 A, since it's a HP psu it has a proprietary connector (6 pin connector). I removed the power supply and wanted to test it using a multimeter. A tutorial I was following showed that I have to bridge the 6 pin connector with another cable and then plug it in to the wall to turn on the power supply, however I plugged in the power supply with no cables bridged or connected to the motherboard and it turned on. Is this supposed to happen? I tested all cables coming out of the motherboard except de 6 pin one and all measured ≈12V.

Not sure if something is broken inside the psu cuz from my understanding it shouldn't be able to just turn on after plugging it in to the wall. Also the psu doesn't have a switch at the back to turn on or off.

Can you guys help me solve this problem?