Convert the recording to a digital format. Be sure to capture a hissy section with no music or background noise.
Sample the portion of the recording with the hiss, then use a software tool to extract those frequencies from the musical portion of the digital recording.
I have done this with CoolEdit with pretty good results.
There are shareware versions of this software online. If interested I will post a full feature version to
http://launch.groups.yahoo.com/group/KennysArchive/ when I get home later today...