I recently started having a problem with my psu, it constantly makes a weird hissing noise whenever i start up my computer, and it doesn't seem to go away even when i lightly tap on the psu in hopes the noise will stop. It all started when i decided to put on a new layer of thermal paste since my cpu seems to be heating up whenever i play games. Afterwards, my gpu (geforce gtx 550ti) didn't seem to get any signal for my monitor so i tried to push it in thinking it was because it was loose, after everything started to work i placed the computer onto my stand but i accidentally switched the on/off button on the psu (while the computer was still on and everything) Thinking it probably wasn't such a big deal i opened my computer again and it says on the screen something about a surge has occured and the system will shut down. When i tried opening my computer again that is when the noise started and my monitor isn't receiving any signal from my gpu at all. What happened? and what do i do to fix it? do i need a need psu or something?
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There can be many causes to this problem. The main one being some sort of damage you caused to the motherboard when you went to work on the internals of the computer through either bumping something, getting residue from the thermal paste in places it shouldn't have been, or static. There does seem to be a lot of cases point towards the power supply however, because if it was not outputting signal to the monitor, it may not have ben getting enough power from it's 6 pin PCIe connector, which in that case means it could be the PSU. Also the hissing could be a capacitor from the PSU that is dying. I would get a PSU from a place you know you could return it to so that you can put a new one in and see what that does for you.
Which PSU do you have? If it is a low quality unit it can cause a lot of problems. Judging by all the problems you are having, along with the hissing noise it could be a bad or low quality PSU. Tell us the brand and model of your psu. However as scorpinock2 said there could be other reasons why this is happening. Reseat your GPU (take it out, make sure the connector is clean, and plug it back in. Also make sure everything is securely attached, i.e power connectors.)