Are you adjusting azimuth on your playback deck?
As for the noise, try to get a sample of -just- noise from the cassette deck while it's playing each side of each tape. Then use that sample as the noise 'signature' for noise reduction on that cassette/side. You can then use the s/w noise reduction tools to remove that signature from the recording. CEP/Audition and other apps will allow you to do this. Know in advance you won't remove it entirely without fouling up the music, but you can reduce it and still have good audio. In CEP/Audition, I've found it best to apply small percentages of noise reduction in multiple passes, rather than a larger percentage all it once.
Likewise, you might be able to isolate the noise via frequency analysis and use a parametric EQ to minimize the noise within a very narrow, specific frequency band.