faint shhhhh noise, pioneer avic-f800bt

The guys wife?

Discconnect the deck output from amp, and turn on, listen if coming from factory amp.

I am assuming you are still using the factory amp,  Try this, remove the radio and at the connection point where you connected the Radio's wires to the harness, cut one of the negative speaker wires.  Play the system and see if that channel still plays.  If it still plays, close the car up and listen to that speaker and see if the noise has diminished.  Start the car and check for engine noise.  If it is quieter and has no noise, clip the negatives of the other 3 channels, insulate all ends of the wires and reevaluate. 

After you clip the first channel, if that speaker quits playing, check for DC voltage on the negative wire on the harness.  The wire that goes to the car, if there is less than a volt of DC, Ground that wire to the chassis.  Make sure you do not ground the wire that comes from the radio.  After grounding that wire, listen for the hiss and also start the car and listen for engine noise.  There is audio on both the positive and negative wires coming from the radio.  If we eliminate signal from one of those wires, it should lower the noise floor.  Only ground the negative wire going to the amp if it has less than a volt of DC on it.  If it has DC voltage you may need to ground it through a capacitor.  100Microfarad should work fine.

I am not telling you to disconnect a speaker.  The vehicle has a factory amplifier.  You are disconnecting the negative input lead of the amp.  Depending on the design of the factory amp, it may or may not play without the negative wire connected.  If it does not play, if you connect the negative input wire of the amp to chassis ground, this will cut the input voltage in half.  This should get rid of your noise.  There is audio on both the positive and negative speaker wires of the radio.  This means that the input of the amp has audio on both wires.  The signal is out of phase on the negative wire.  That is how a High Power radio works.  If you ground the negative inputs of the amp, this will cut the signal getting to the amp in half, thus getting rid of the noise.

You are correct in the procedure for checking for DC voltage.

Depending on the design of the amplifier, it may decrease sound quality if playing it without the negative wires connected.  If the sound is not right, ground the negative amp input wires to the chassis. 

There is a good chance that the factory radio only has signal on the positive speaker wires.  Your radio has signal on both wires.  Twice the signal = twice the hiss.