|"is it due to ram or virus or hard disk problem"|
No. is it due to a buildup of dust and dirt on the fan blades and possible wear & tear on fan bearings? very likely.
Your question should be " How do I safely clean out the built up dust & dirt from inside my PC case"
Begin by disconnecting the power. Open the case. Take a small paintbrush and gently dislodge the dust & dirt from all fans and heatsink. Use a can of compressed air and blow out the dust. It is best to do this outdoors.
If cleaning off the fan blades does not reduce the noise you should consider replacing the noisy fan as a worn bearing can lead to fan failure & system overheating.
Goin' Fishin' (Some day)