For WVR, i dont think there would be any noice in pilot's visor since Both IR and Optical seekers are actually passive seekers. Initial versions of Sidewinders did have active/semi active radar seekers, but newer generations utilitzed IR/EO seekers due to improvements in technology. So unless an sidewinder is not fired and detected by MAWS, i dont think pilot should get a beep in his head. For radar guided threat MAWS/Radar/RWR can be utilized. Radar Warning Reciever continously scans for active radars in the area by receiving and analyzing received signals.It then matches the recieved signals with frequencies in its repository and gives a radar warning to pilot. Since it is a passive sensor, so if LPI technique (sending different wavelenght at every emission) is used or Radar is switched off, it cant do much (or anything). The sudden pick up in beep is actually when MAWS dectects a missile approach, establishing a lock doesn't mean a pick up in tone (Since there isn't a pick up in radar frequency or anything if an aircraft is acquired by fire control radar). So if I approach a plane from an angle where he has no radar coverage, switch my radar off to cut singal supply to enemy RWR and lock the enemy aircraft using a passive seeker (IR/EO) enemy wont even know till the missile launch that there is someone nearby. Thats just a part of story. EW and other techniques also perform the same role by fooling enermy RWR and Radar about existence or location of intercepting aircraft.