asn expander is the direct oposite to a compressor (kind of) the compressor will take signals above a destignated level and reduces the volume, where as an expander lowers the signal BELOW the designated level, kind of like a gate but with out the direct switching off of the siganl..... so how then can one dial be called an expander/gate?by the time you expand to the point of silence, you can continue into negative silence, or if you like gating...
any how, i'm pretty sure thats what it is, or something very close to that, not an easy one to explain, but i hope this helps a little.....then again i am probably entirely wrong....
alex to answer your question, yes, it will reduce noise.