I agree with Dumbluck - this sounds as if it has already been worked with somehow. FM can be patchy anyway, it reeally depends where it was picked up, how, and what the frequency details are from the broadcaster. It might be worth your while looking for a pre-FM tape. In my experience, if you start working with FM you quickly lose what is there. The only thing you might be able to do is add some depth. Killing the hiss will probably take a lot of the higher dynamics with it. But it doesn't do any harm to experiment!
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