Pat,
The idea is that it starts up slow if low flow is required and then goes to full volume as flow rates are increased. The "regular" pump sounds like it's just either on or off. However, the ShurFlo technician I talked to told me that ALL of their extreme series pumps are variable rate pumps. Particularly, if you have a captive water pressure tank like most Hatts have, it doesn't matter which pump you have because the captive pressure tank gives you water pressure at any flow rate.
My old SmartSensor pump used to make beeps and boops sounds as it kicked in and again as it began to shut off, probably as the different pressures were reached. Pascal has one in his 53MY and it makes the same strange sounds. The 5901-4212 just runs fine when the pressure drops and stops running when it reaches 50 lbs/sq in again. I think you picked the best one to begin with (the 4212) which is made specifically for 32 volts.
Doug