If the noises are very frequent, I'd advise you to use the stethoscope method to determine the exact source. If the HDDs are physically reachable, use a thin plastic hose (about 5-10mm diameter) to listen. One end in your ear (make sure it does not come out from the other) and put the other end to suspected surfaces. Even a screwdriver used in a similar fashion would produce amazing results (sharp end to the device, rounded end to your ear)
Any HDD which produces strange noises is an excellent candidate for failure and must be replaced asap.