have you got the new drivers installed? 7373 released on 5th Jan
If not, get it installed
if it happens with the latest driver, uninstall them and try the windows default driver
only reason i mention that is quite a few people had a number of audio issues with the last few audio drivers across the maximus range of mobos, which asus said they would be fixing in a future update
I'm on the Ranger, so I'm still stuck with the 7326 driver and I get a pop at boot up and shutdown (plus other issues)
Until the issue gets resolved, I used this to fix it ->
https://www.youtube.com/watch?v=MZyvOUdbUqY&feature=youtu.be&t=4m1s
i7 4790K - ROG Maximus VII Ranger - 8GB Kingston HyperX Genesis - Asus GTX 980 Strix - Corsair H80i - NZXT H440 - XFX 850W Black Edition 80+ Gold - Win 8.1 Pro x64