In the crew rest area behind the flight deck on the B744 a lot of the noise is aerodynamic combined with conditioned air flow noise and therefore steady.
Agree 100% with ChristiaanJ, steady noise not a problem, intermittent noise varying in type and volume is a disaster for anyone trying to sleep. Ear plugs that are so good they can keep all noise out won't allow fire and smoke alarm noises to be heard either, (pertinent to Hotels rather than aircraft!).