but how could it be the hard drive?
Anything with a motor can make those noises.
Note you should be opening your PC every month or so to inspect for dust buildup, and cleaning when necessary. So pop off the side and look inside. If there is a heat-trapping layer of dust in there, it needs to be carefully cleaned out. If cleaning is necessary, unplug from the wall and touch bare metal of the case interior to discharge any static in your body before reaching in.
The easiest way to pinpoint a fan noise problem is change the fan speed. Touch bare metal first then carefully touch the center hub of the fan, just for second. If the bearings are making noise, touching the center hub will change the rotation speed and the pitch of the fan noise will change too.