Noisy cpu fan

Hello,
My pentium 4 3.06G cpu hs fan has been very noisy now a days. when it spins in lower speed(1000-2000rpm), emits a very high pitch sound. but if it gets above 2100rpm the noise becomes low frequency but loud metal sound. i want to get out of both those two problems. and i dont want to change it anyway. please suggest remedies to be applied on that existing system. thanks in advance!