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.
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.