Open up the computer and make sure all connectors and add on cards are seated properly. That means straight/level/flat in the sockets. Especially the ram chip(s).
Restart the computer and press the Delete key on the keyboard for about 10 seconds. The bios screen should appear. Now look for a setting in the bios menu that says something like Optimized Settings, usually on right side of the menu. Select one of them if there is more than one you may want to try all of them. This will basically scale the system back to minimal factory settings and then save and reboot.
If it still makes that nagging noise, there are several possiblilies. Bad RAM, Bad Video, or heat sink isn't making good contact on the CPU. Baring those, I would take out any add on cards and disconnect all drives. Get the motherboard, ram, video working and then add on one item at a time. The one that makes the nagging sound is your problem device.
