tech questions for those who know alot more Than I

["Christopher White" <magicicada@mindspring.com>]

i am recording to marantz 4 track cassette Pmd 740 tape machine and well
when I do playback after i have recorded i get kinda a crunchy noise ridden
nasty nasty sound when the *music* gets  loud. Now i am not red lining it 
to
hard so I am not sure what this is could it be that i need to compress it
more when i record to 4 track? also on track 2 when i tweak my mid eq it
begins to make this analog synth sound like "WHOOO wee Whooaa ooommmm" but
it makes no sound when i keep the eq up high. what an ass pain this is!
please offer me suggestions and i will be happy to try them out.
thanks alot
c.white