Hello Alex and welcome to PCHelpForum.
That dont sound good.
Are you sure all fans are spinning and the noise maybe
CPU or graphics card?
Do you get any picture on your monitor?
Please check your processor heat sink is seated correctly you may of moved or knocked something while you were inside your computer.
Have you tried to rest the BIOS via reset jumper located near BIOS battery?4 pins with a little plastic jumper on two of them.Move it to the other two pins for a few minutes then replace on the origonal pins.Or remove BIOS battery for two mins replace reboot see what hapens for me please.
Look forward to your next post.
JB.