The sensor has a ratiometric output.
If, at a certain pressure, the sensor outputs 2.5volt when powered with 5volt,
then the sensor outputs, with the same pressure, 2volt if the supply drops to 4volt.
Did you actually trace the cause of this pressure change (with a scope), before trying to fix it.
An 7809 for two pumps and the Nano sets off alarm bells in my head.
That regulator needs at least 2volt across to work, and will get into foldback current limiting above 1.2-1.5A.
Did you measure the output of that regulator with a scope (for dips).
I assume you also connected supply and ground of the sensor directly to VCC/GND of the Nano.
Sharing VCC/GND with motor wires could also give this behaviour.
If the vibration is purely from air pressure ripple, then try a mechanical solution first.
Stuff a piece of cotton from a cotton swab into the short tube that goes from the T-joint to the sensor.
Tightly packed inside the line, and close to the T-joint.
That will give some air flow resistance, and some buffering of the rest of air in the tube.
What do you mean with 'remainder'.
You could measure/calculate in integers, so you don't have a remainder.
Leo..