2013 Tahoe 5.3 hiss on startup

[Ex Euro Driver]

Got a strange hissing noise on startup and then goes away after warmup. Sounds like it's coming from water pump pulley/center of motor and is rythmic. Goes away after 5 minutes of driving. It's not either of the idler pulleys. Any ideas?

 

[Ex Euro Driver]

Anybody?

 

[63Blackseries]

It could be a vacuum leak, i also have a hissing noise on the driver side and you can hear it clearly on hard acceleration but i haven't found whats causing the noise yet.