Could be pipe run length, but vibrates only in the up.

Isolation around feedline that penetrates floor or walls;
How does the motor/pump sound in the machine room?
Isolation at the piston to car? Isolation coupling in the feedline and a muffler?
Jammed feedline connection transmitting that harmonic pump vibe?

Is the ride spongy at all? Bouncing at slowdown points or final stops?