If you have to ask yourself if your computer is too loud or if it should be making that noise, I would go ahead and say, no, it shoud not be.  Computers these days are fairly quiet and if they're making a lot of noise, it's not a good sign. 

Hard drives and optical drives can also cause excess noise.  As a hard drive ages, it will start to make noise when accessing files and during virus scans and other times of heavy usage.  This is normal, however it is a sign that you should start backing up your system and it might be a good time to contact a computer repair technican to replace your hard drive before you lose information.  Your optical drive is also succeptible to dust infiltration. Luckily optical drives are inexpensive and easy to replace.