few things, with side panel off, does sound seem to come from were the slave harddrive is?? what brand is the save drive (maxtor, seagate) have you went to manufacturers website in troublshooting area?? if its clicking and from harddrive its possible its just doing alot at once, if whining or groaning from harddrive area, its possible the bearing in middle of platter is about to go, its also possible that the noise you are hearing is a cpu or power supply fan, thats what happened to my dad's xp, turn it on it will make a loud groaning noise.
if the noise you suspect is sounding really bad and from the slave drive, back up the important stuff onto a usb or cd format or migrate to the master so there would be less impact if and when it dies.
good luck
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