The only thing I can think of off the top of my head is that it might be noise from the motherboard/on-board audio itself. I know that the front panel jacks on my computer have much more background noise in them than the rear-panel jacks, just because of all the case interference that comes through the long wires that attach them to the motherboard.
Do you have different jacks that you could try or an external sound card? Also, I suppose it might be that the Y-adapter itself is bad. Do you have this issue with other headsets/have another headset you can try with the motherboard or y-adapter?
I’m not really a tech support guy so beyond that you’ll have to get in touch with Corsair!