My neigbhor installed a high efficiency furnace using sidewall venting with 3" PVC pipe. The pipe is about 3 feet off the ground and terminates with a 90 degree elbow pointing directly at my house (patio area). The problem is that the vent emits a high pitched whistle when the furnace is running which is very annoying. Would it be okay to add another elbow to point skyward to redirect the sound (and vapors) up?

Are there any other suggestions to dampen the sound or prevent the whistle?

Pointing up is probably not allowed in the install manual since this just makes it a funnel to collect water with. Now sideways should work. If the neighbor is a nice guy work with him if not call the local building department.

He may not have taken a permit for the replacement furnace and I am sure the building dept will enjoy busting him. LOL

There are usually noise abatement rules on the books in most building departments. They will make him fix it.

Last call lawyer up go to court and win.