noise from computer, fan over processor or smps ?

i have a desktop computer. last few days i am hearing a noise from my c.p.u. i did checked it and clear all the dust etc from fan over the processor , the sound occurs for approx 1-2 min. only when i used to switch on the pc.after it itz okay. what should i do? is it from that fan or from s.m.p.s.?