Acer Nitro AN515-51 fans keep speeding up

Hello everyone.

So I have an issue regarding my laptop. My Acer Nitro 5 (AN515-51) seems to keep speeding up when its idling, both when the screen is on / when it turned black. This happened even when there are no apps opened when it goes idle, and at one time it speed up just after I left the laptop a bit after I restarted it. I tried downloading Predator Sense to check how fast it went. At one time, the rpm reach 5000-6000ish and the heat hit 75-88°C at it highest. If kept left alone, the fan would just keep speeding up to the point I can hear it clearly from a feet or so away, until it just suddenly went to halt / slowed down enough to not make any noise. But it'll just speed up again after a while, and keep repeating the speeding up-halting cycle.

This issue never happened when I actually used the laptop, even when I'm opening multiple tabs of browser while using another heavy load apps such as video editor simultaneously. At most when I'm using it, the rpm would hit 3300-3600 with stable heat at 56-65°C. The only time it'll speed up is when I'm using it for playing heavy load / processor demanding games. But even then, the laptop would slow down in a controlled manner after I'm closing the game.  There is also a curious thing in which if I use / wake up the laptop when its idling and the fan is currently in fast speed, it'll slowly, but noticeably, slowing down until its normal again.

The current condition of my laptop itself would be these: I plugged the power most of the time (I plugged it out when the laptop is shut down). The battery status at Windows Mobility Center is set at High Performance. I have tried uninstalling any unnecessary apps that might put more load in the system background. I've updated both Windows and Nvidia to it current latest version. I've checked if there is anything wrong with battery etc. using Acer Care Center, no problem there. I also tried to air dust my fans. Before the dusting, I noticed there are some hair-like dust in the fan intake vent and cleaned it. After dusting it, some small particles and dust(?) did come out, but the speeding up issue still persist.

I tried to research this issue, and some suggest undervolt, in which I do not want to do since I read that it'll reduce the lifespan of my laptop. I also saw somewhere that unplugging the power help to slow down the fan since the laptop can't run at full capacity. It did work, but it seems like only a temporary fix and would rather be an inconvenience to do that everytime the issue arise. And it won't fix the issue that when it is plugged up with power, the fans will speed up again anyhow when went idle and might lead to even bigger or worse damage to the laptop if left alone. This certain issue only started for several months ago, if I remember correctly.

So, does anyone have any suggestion on what might caused this, and how to fix it? Or should I went to repair center?
Still if I do go to repair center, it's more reassuring to know what the possible cause of this issue beforehand as to not go there blind, and got the issue unresolved in the end.

Thank you.