The potential noise. It could be the pump, but I have since learned that it is submerged and is pretty quiet. It also could be the water sounds???
Yes, it's the sound of water that can be loud or at least be heard by neighbors. Many people, especially who like ponds, do like the sound of water. However, some don't. Much more importantly many people do not like a neighbor who disregards their rights. Paying kind of lot for a house, home being a castle and all, and every time they go into their backyard they hear the water it just reminds them of a neighbor disrespecting their space. Wind chimes for example can cause a lot of trouble.
What I do is talk with neighbors, explain what's being built, answer questions, tell them if there'is ever any problem that I have absolutely no problem fixing it. Give them power over their space. People are nice when they're not being disrespected.
I never had a problem in San Jose. Here in Phoenix there are a lot of very right wing, self centered, non-thinking self-righteous people who think they should be able to do whatever they want and to heck with everyone else. It often creates serious wars. We have a lot of HOAs here because most people don't want to live next to these types of yahoos. I'm sure you're not like one of these yahoos but your neighbors may be concerned that you could be due to a bad experience where they lived before.
The HOA board members only represent owners. If any single neighbor objects that should be enough to sink a project. It's an HOA, not a democracy. Good luck, I'm sure it will be fine.