When I start my computer, the fans are loud
Computers generate a lot of heat, so the cooling is of vital importance. The fans in a computer need to remove a significant volume of hot air, and they can sometimes be very loud, especially when the CPU is working hard. As the CPU "throttles up" to 100% usage, it uses more power and generates more heat. Sensors inside the computer can detect the temperature increase, and speed up the fans.
Using less CPU to reduce fan noise
If your fans are running loudly, you can try running fewer programs on the computer. Do you have multiple browser tabs open, playing video or running advertisements? Try closing any you don't need, or closing the browser entirely if you're doing something else. Reducing the number of running processes will lower your CPU use, which may throttle down the fans.
If you think there's a problem
If your fans never quiet down, or if they're suddenly a lot louder than they've ever been, this may indicate a hardware issue. Fans often get abnormally loud when they start to fail. If you suspect there's a problem, don't ignore it — if the fans fail, your computer may overheat and shut itself down. Replacing a fan is much cheaper than replacing the CPU!
Replacing the fan
If your computer is under warrantee, contact your manufacturer about the problem. Otherwise, you may want to bring the computer to a local repair shop. Or, if you are comfortable working inside your computer, you can open the computer case and examine the fan's operation visually while it's running. If it seems like it's failing, you can replace it.
Before purchasing a replacement fan, consult your motherboard owner's manual for its technical specifications.