[jacquesm]

That sounds very much like either tower thump or a broken bearing, I think the neighbors would have a better case if they pushed the safety angle because a turbine in a bad state of maintenance is dangerous.

Then they'll be forced to fix it and it will be quiet again. You can ask them if it always was that noisy, if it wasn't then that's an extra arrow in their quiver. I'm very much pro renewables but safety is a major concern and operators that do not work safely and/or ignore valid complaints are a net negative for renewables.

[wood_spirit]

In this case it is the terrain. The sound is funnelled somehow. In the planning process the owner has to commission a study that reports if the terrain shape is ok and compute the noise on nearby houses etc. Apparently you can get what you pay for. In the appeal a more independent study pulled the original study apart.