What year Mustang? what type of pump are you using now and where did you locate it? I ran 2 external fuel pumps (holley blue in the rear, & red up front) on my older mustangs, and used a thick piece of rubber between the frame and the pump bracket mounts which made them hardly noticeable at all.
Same here. Just replaced my old pump with a new GSS340 255HP Walbro, and it's also very very quiet. I've read many post's about 255's being loud, makes me wonder if it's the brand or the way it's installed.. who knows for sure..