I'm not sure of the sound being referred to here. Is it a noise generated by the hardware (like fans spinning, HD noises etc.) or a system noise (system sound effects, like Submarine, Blow, Funk, and the classic, Sosumi)?
To help, we'd need to know what you were doing at the time, when it occurs, perhaps what apps are open etc. We can't really answer any questions unless we know what's going on.
It's like asking a doctor "why am I sick?" and not visiting them, or saying anything more