How does the fan get louder when I close apps?

That can happen without anything being wrong. Fans respond to temperature, which takes time to fall after an app closes. The app may also still be saving or syncing, or Windows may start background work once the PC becomes idle.

Open Task Manager before closing the app, sort by CPU, and watch what happens afterward. If another process becomes busy, you’ve found something to investigate. Also check whether the app’s process remains running after its window closes.

If CPU activity drops and the fan settles within a few minutes, it’s likely just cooling down. If it stays loud while the laptop is idle, check that the vents are clear and that the manufacturer’s fan profile isn’t set to Performance. Try Balanced mode rather than forcing the fan speed down.