Sadly, this is the very issue that I had with my new one. I tried another phone on my carrier (sprint) as well as a new One on another carrier (AT& T) and that hissing was still there. I started a thread about this, as well [threads now merged]. It did not receive much attention so I assumed that it must be me or a rare few.
However, this was not to be the case. Over on the XDA forums there is better coverage of this issue and lots of folks are reporting in on this. What is not clear is if HTC has acknoleged the issue or it is still not confirmed by them. It is a good source of information.
I plan to monitor this thread for a firmware update and/or a resolution. I returned my One and will not get another until this if fixed.
I hope this helps you. It is frustrating. This I know, for sure.