this could be many things and hitting it wont help. it could be any of the pcs fans either slowing or catching on something or it might be that your hard drive is on its way out. either way it needs looking at so if you dont feel confident to try it yourself get someone else to do it for you. check everything and i mean EVERYTHING.. connections, wiring, the lot

Maybe a piece (internal/external) vibrating against the case?

Try to disconnect fans. Block the PSU one (for a secs only) before your PC starts.

Move your PC to another place to see what happens.

Disconnect disks, one at time.

ok,thx for advice.

Quite often CD-ROM (and other optical) drives that are failing start to sound like airplane turbines spinning. Does the problem seem to go away when you press the eject button on your CD-ROM drive? If so, replace that drive. Otherwise, it is most likely a fan, and the previous posters' information should help you resolve the situation.

-Matt
Quick PC Repair (Serving San Diego, CA)
www.quickpcrepair.com

The problem is still not fix so far,and i dont think it is the cd-rom,and the fan has been checked,no dust no object messing with it.
I usually hit/punch/slap the upper front part of the machine and the noise will stop until the next time i turn on the computer.
Can this be temperature related problem?It is winter in my country(australia) and very cold.

the upper part of your PC huh!! this is where your CD drives etc should be. Have you checked to make sure that your CD drives etc are secure in their bays and does the noise only occur if you have a disc in the drive.

Matt is right by what he said. sometimes the internal mechanics of the drive can sound like aeroplane turbines on failing. this could be a sign that the spindle might be coming loose. try another cd drive in replacement of the one you already have in there and see if you still get the same noise from it. i dont think you will.

hi again and sorry for this stupid problem.
Unforetunetely i do not have cd driver that can be use for testing and matt asked me if the problem would be fix if i eject the cd driver and my answer is no and that's what convince me it is not the cd driver 's problem.
anyway,when the noise first started,i hit the back of the comp(where fan is located) and it was only recently i change my target to the front (since i was too lazy to hit the back) and both way worked,and in fact hitting the back only take 1-2 times and hitting the front takes 4-5 times until it stop.
Please tell me if you have any idea of what is going on,and if it is really the cd driver i would buy a new one. thx very much for ur time janine and all other pros

no problem Tom.

without actually looking at your PC its very hard to diagnose what the exact problem could be. If the noise appears to be loudest at the top of the machine it could lead to one of two things, one of which i've already said before in it being the cd drive. the other option here is probably the Power supply unit. it is quite possible that the power supply fan has come loose or got a chip in it somehow.

the only way to tell whether its the cd drive or the PSU is to do the following.

1. power down the machine and unplug the cables. ensure the wall outlet is off
2. open up your machine
3. locate the bay where your cd drive is located
4. gently but firmly remove both the IDE and power cables
5. power up the machine again (PCs dont need a cd drive to be able to kick in)

If the problem is still occurring after this reboot without the CD drive connected then i would most certainly rule out the cd drive as being at fault and concentrate on the possibility of the power supply unit.

If you dont feel confident in yourself in carrying out the above proceedure please take it to a repairs store.