Exchange bathroom fan
The Panasonic fans are an excellent choice.  They are quiet and very efficient.  Broan/Nutone also makes excellent products.  I would recommend the new low sone models. Bath fans have a definite life expectancy. As they get older the bearing in the motor dry out and the noise level go up, up, up.  Either of these would make a good replacement; minor ceiling modifications may be necessary.  Be sure to vent it to the out of doors.  Because of the low sone quality of the new fans, I doubt the the reduced speed feature will be necessary. I would not recommend trying to incorporate the reduced speed switch with the new fan.  It probably would not be compatible.