I've used SoundSoap for a long time and I'm very happy with it. You have to find a 2 sec sample of just the noise you want to eliminate (easy on a cassette tape) and go from there.

Originally from Bias, now defunct, it is still supported by Soiundness. Versions for Mac and Windows. I'm on a Mac and I bought it from Bias a while ago. Never regretted the expenditure, except to note that the last time I looked, it was bundled free with Toast, CD burning software which I've also used for a long time.

I think you can get a free trial.