A pressure sensor is ratiometric, meaning it's output is a ratio of the power supply.
If the power supply of the Nano varies (USB ripple, etc.),
then the output of the sensor also varies, and Aref of the A/D varies.
But all those variations cancel each other out , so the returned A/D value stays the same.
Unless you stop only one of those variations (with a capacitor/filter).
Then the variations are not cancelled out anymore.
So the R3/C5 filter you added here is a bad thing, that makes A/D variations worse.
Leo..