When I switch on my computer some sounds come inside from my cpu.. it sounds like a juice machine.but after I log on the sound will disappear and run perfectly.can anyone tell me how to avoid these sound

The sound is most likely your fan powering up or your hard drive. You can try having first the fan, then the hard drive replaced.

Also you can try a good cleaning of the inside and outside of your computer. You can buy cans of "air" to blow it off just be careful around your components and do it in a well ventalated area.

Sometimes Graphics cards will start with the fan running at full speed. Once inside the operating system they get the info they need to regulate the fan and it slows down. Why they do not get that info at boot up can be caused by a number of things. One would be the improper power connector to the card like using a six pin motherboard connector instead of the Pci-e. Could be the boot up information needed got corrupted and reinstalling or updating you drivers could help. If you installed software to control your cards temps and fans other than the manufacturers they could interfere.