Strange High Pitch Humm

I was troubleshooting a co-workers coputer trying to find where this high pitch humm is comming from. This humm comes around two to three times a day when the computer has been running for awhile. I opened the case and waited for the sound. When It started with the high pitch I looked and looked and found a speaker on the bottom the the tower case that is causeing this noise. It is pluged into the mother board and the only marking on the motherboard says speaker. This computer was put together by another person and I do not have the boards paper/specs. I was hoping someone might have had this problem before or is this just a case of unplugging the speaker. The computer is works fine and notheing crashes or stops when this sound comes on. It is just annoying. Any help would be great. Thanks.