I'm in the process of cleaning mine at the moment.
Done about 10 so far and they sound ace.
I use cool edit 2000 .
1. In the main view highlight a small section off just hiss at the start or end of the tape. (important its just hiss)
2.Go into the transform menu / noise reduction . Click th button onthe top right which says get profile from selection. - Then click close .
3 In the main view highlight all the tape start to finish.
4. Go back into noise reduction and this time click ok .
This may take 30 mins or so .