You might be confusing "hiss" with like a simple background "white noise". You will always hear white noise in a speaker when an amplifier is turned on--and since a headset has the speakers right up against your ears, you might be confusing this with "hiss". If you've got really sensitive ears, then I can especially see this scenario. However, it might just be normal because I've heard amplifier activation (i.e. white noise) in a few different manufacturers' headsets--not just Turtle Beach.
If you hear the same sound in the replacement headset that you're getting, a couple things you might want to try are using new batteries that are a different brand. Also, there's a chance that your electricity may have some sort of a ground loop that's causing a hiss. (If the "hiss" is literally loud enough to be heard over game sounds, that is). The best thing to do is try hooking up the headset on another source that is completely separate from your gaming setup. Trying it out in a friend's house would be even better just to be sure. Oh and, use a different USB port too so you can see if the USB power is clean. The best way to do this is connect the transmitter's power to a laptop that's running on an internal battery.
