Thanks for the further information.
The reason this happens is because the computer is running hot and it is trying to cool it self off. Sometimes when you first start up or come out of sleep for example you may also hear them rev up but then they will usually settle down.
If they are running loud and a lot there could be a few reasons.
First, if you are running a lot of programs or you are working on a lot of data on a large document or playing a game the processor and components will get hot so the fan will speed up to cool it. You will have major issues if it does not cool down, your processor could get fried if it is not cooled properly.
Another thing that happens often especially on an older computer is it gets filled with dust. The dust blocks the air vents and can get in the fans as well causing them to have to work harder.
A few things to try to correct are, first try running less programs at once. Look to the bottom right near the clock and see if there are a bunch of programs that you may not need that you can turn off, usually by right clicking them and choosing to quit.
If you do not have many or that does not help you are going to want to get yourself a canned of compressed air like this:
Any store that sells office supplies will sell this.
Take the compressed air and blow out all of the vents on the computer itself. If you feel up to it remove the side panel of the computer and blow all of the dust from the inside and around the fans.
Without seeing your computer in person it is hard for me to say 100% what the issue as however my best guess would be that it is just dusty inside and not getting the proper flow of air.
Also if the computer is kept in a space like under a desk make sure that nothing is blocking the vents.
Usually a good cleaning with canned air will help it run cooler and quiter.
If you need further help please do not hesitate to reply back and let me know and if you have a moment please remember to rate my service.