First off hard drives occasionally make a noise, it is just going to happen once in awhile since it is a rotating platter on the inside. It's when the noise is persistant like you said, that you have a problem on your hands.

Sometimes the noise can mean a part just broke or malfunctioned on the inside, so make sure you backup your data as soon as possible because you might only have a little time left until it stops working completely and you might lose everything.

It could also mean your hard drive is on the verge of breaking, but you still have many months of use until it cannot be used anymore. Either way, a persistant noise is not good, so when you hear it be prepared.