hard drive boot priority?

My dad has been having problems with his computer, when he puts it on the processor sometimes makes a clicking noise and doesn't load up. One time when it was working he went into boot priority and wrote it all down to see if anything disappeared when the problem occurred again. When the pc is working, there is a long code beside hard drive boot priority that starts with WDC and when it is not working the code is gone and in its place it says "Boot-able Add In Cards" has anyone any ideas what is causing this and if we can fix it ? sorry for the lack of information. thanks