Where does that hiss on the master come from?

Are you talking about analog tape hiss? Common from the early days of stereo (listen to any Mercury Living Presence record or CD and you'll hear it), and an "evil" of analog tape vs. digital tape (one of the aspects of digital recording that many felt enough of an improvement over analog to overlook the flaws of early digital). There are some programs available to eliminate that noise from a master tape; some remasterers try to eliminate it, others don't because they feel it compromises the high frequency content of the music.

That's analog tape hiss (noise) that you are hearing. It is inherent in the analog recording process and can't be totally removed from a remastering either for analog or digital. It is the noise from the tape transport and electronics that gets "recorded" onto magnetic tape along with the actual musical signal that you are hearing.

It is noise inherrent as a result of the electronics (recorders) of the day, the tape formulation, the tape width, and the tape speed. Early studio electronics also had lots of noise of their own that aren't attributed to the recorders. Early noise reduction (Dolby-A) was heralded as a breakthrough in reducing tape hiss. It was further advanced with Dolby SR that all but eliminated tape hiss. A modern recorder such as an overhauled Ampex ATR running at 30 ips with modern tape formulations (Quantegy GP-9) have eliminated tape hiss and don't require noise reduction schemes anymore.

This noise cannot be removed nor would you want to try. Removing the noise would be about the worst thing you could attempt to do to the master as it would remove sound content as well.

South Philly. No, really. Hissing occurs in the upper registers and usually means the recording has beeen mostly left alone allowing a more natural sound in that region. Vocals are more natural, symbals ring truer, an openess occurs surpressed by Dolby treatment which also introduces phase shift. Usually more dynamic too. I'll live with the tape hiss as opposed to the un-natural sound of processors.

If the master tape has been digitized there are processors/software that can reduce (3 to 10dB) the sound of tape hiss without harming the music signal. Just a small amount of noise reduction can make a big subjective listening difference.