Noise from ac behind front seat occationally sounds like a police siron
it sounds like the bearing in your rear a/c fan blower is worn out.
or something crawled in there and is screaming for help 
+1
In any event, I’m still trying to figure out what a “siron” is.
The expansion valve for the rear AC is defective.
The noise is from the refrigerant passing thru the bad expansion valve.
Tester