Some 255s are pretty loud. It's a Luck-of-the-draw kind of thing. Other's are quiet but are isntalled less than perfect.
If the pump or bracket comes in contact with any portion of the tank then it'll resonate off the tank. I tell you this because I've installed one or two that were loud as hell. Bad enough that the whole assembly was pulled apart again to see what was the matter. Often, just reinstalling it and getting the little rubber or-ring around the cap perfectly positioned, cured the problem.
Later, I got into the habit of slaving in a piece of fuel line between the pressure and return ports and feeding power to the pump prior to reinstalling the tank. Just to check the noise before I wrestled the whole damned thing back up into the car.
Just ensure that everything is connected correctly electrically if you choose to do this. You'll never have a better chance of blowing yourself up than you do with an almost empty tank, lots of fuel vapors, lots of air, and a spinning pump than you will when you do this.
--and--
DO NOT DO THIS BY APPLYING POWER DIRECTLY TO THE PUMP TERMINALS.
Install long wire leads to the terminals and connect/disconnect power to teh pump with 15 foot (or more) wires AWAY from the tank.