Some hard drives do it when they get older. I have a Barracuda somewhere that sounds like a cement mixer whenever it reads/writes. It doesn't throw up any errors and appears to be perfectly healthy, but my god is it loud. It was silent when new, it must be over 6 years old now.
My hard drives in my current PC make barely audible read/write sounds now after 2 years, when they were first put in, they were also silent.