PC extremely loud when running

I have a dependable PC for a few years now and currently it is running extremely noisy. The fan use to slow down but now it runs ongoing and at maximum speed. I ensure to maintain my computers by keeping them clean. I am conscious of what I download. Because Window has so many processes running I believe that it slows down the computer. I have cleaned and dust the computer inside out. I want to keep this machine to give to my family member so I really need your help. Thanks in advance.
