Help with my Headphones

Hey guys, when i bought my iPod about a year ago they came bundled with the piece of crap they call headphones so i recently decided to buy some nice ones, they cost me £18 and i went for the Sennheisser CX300 headphones, i plugged them in to my iPod and they were great, no problems what so ever, but then just recently i have noticed a big problem and i cannot send them back because they came in one of those plastic cases that you have to cut open to get into.

My problem is that the headphones now have a slight hiss in the background and when your listening to a song quietly it really can be quite annoying. Now i thought it might just be my iPod so i plugged them into my new Sony Walkman MP3 player i got under a week ago and the hiss is even worse, i then turned the volume up and down on the sony and this hiss remained the same level of loudness however when i paused the iPod when these headphones are connected and then just turn up the volume without a song playing the hiss gets louder and louder and makes a weird sound in the headphones, they never had this hiss problem when i first got them and i never played my iPod or Sony at max volume with these in, i just bought these and i now see them as a waste of £18, can anyone help me with my problem?