Where is anti siphon on 3295

[illapah]

I recall reading about this but after searching, I couldn't find the answer.

Where is the anti siphon valve for the black tank flush on the 3295? I think mine is bad but don't know where and how to locate it.

Symptom - kind of a squealing sound when trying to use flush and do not hear the volume of water going into the black tank that I heard last year.