The 1/4 horsepower Gast 0523 specs says it's less than 70 dBA. That's about as loud as a dishwasher. 60 dBA is the volume of common conversation. I can run my pump in the shop on the bench and talk to someone without problem. The spec states 180 watts as you mentioned.
So, I'd say that they are not loud UNTIL you are at the pond in the dead of the night with no other sound sources, then it's all you'll hear.
I am building a small outhouse looking structure (out of scrap stuff) to house my pump. This should help cut the noise down as well. And, most of my aeration will likely be at night due to the small size of my pond.
