We have the 40E, but it's essentially the same pump....
Ours sounds like a hammer when there is air in the feedwater. I had ours mounted with the pump below the motor
(i thought it made sense to put the saltwater below the electrical
stuff) and it sounded horrible and product water
output was dismal. I flipped it 180 degrees, in this orientation it was able to purge all of the air in the system and is remarkable quieter and produces more water.
When I try to use it underway the location of the pickup water pulls in air bubbles and it again sounds horrible. I'll be putting another thruhull in the lower reaches of the bilge