When your hard drive is continuously spinning it could be caused by a couple of things. The first and easiest thing to do is to run a disk defragmenter on the drive (not applicable for SSD drives). When a hard drive becomes fragmented it has to search the disk in several different places that are spread out to get the information rather than in just one contiguous section. You should check it about once a month to see if it needs to be run.
Another reason a hard drive will be constantly working is because you do not have enough RAM in your system and the computer has to go to the hard drive for memory to make up for it.
I would also check your startup items to make sure you don't have any unnecessary programs starting with your computer using up your resources and running in the background.