Have you tried swapping out that 1/8 -1/4 adapter? tried plugging in some regular earphones? sounds like the noise could be a contribution of a bad plug somewhere....otherwise try a DI, most people have a few hanging around, if you don't have an extra one, run to a nearby store and just pick up a cheap one...I like the art stuff personally (since trend is me need something, me buy something....) its treated me well...I've got 2 of these
http://www.artproaudio.com/products.asp?type=90&cat=13&id=104
you could also try something like this that meant for AV stuff
http://www.artproaudio.com/products....&cat=13&id=131
I run both my laptop and the ProPresenter imac from 1/8 - stereo RCA's and a 1/8 - 1/8 extension - 1/4 (the port on the mixer has RCA's and a XLR/1/4 combo jack...this way I only use one channel out of my 8 for computers...)