Mike,
Forgive me if I'm saying something you already know.
Motors are inherently electrically "noisy" devices, especially those running at high amps and RPMs.
Before anything ensure your power cable to the motor is not running close to any signal cables.
You have two areas to tackle: 1. Reducing the source of the noise and 2. Shielding from noise.
1. Have you tried soldering a capacitor across the motor terminals (see C1 in diagram below). Ceramic not electrolytic. You can also use two in parallel of different values to reduce a wider spectrum of noise e.g. 0.1uF and 10uf. Remember frequency F = 1/(R x C), R being your pump motor's impedance.
Ideally put them as close as possible to the terminals.
Additionally, you can do the same with 0.1uF from each terminal to ground (0V) - see diagram C2 and C3.
Finally you can use heavy duty inductors in the supply lines to pump motor - L1 and L2 in diagram.
As an experiment I put a 2Farad capacitor (which is HUGE and used for high load sound systems ) across my Z battery terminals and found idle became smoother and also revving out keener.
2. ensure your signal cables are tightly twisted together e.g. like network cables ...
Also an easy win is to use shielded twisted pair cable from sensor to anything else.
Hope this helps a little.