Panaflow fans are amazing, I've been recommending them for years!
People used to laugh at me in THIS VERY FORUM when they would ask "what kind of fan" and I would tell them "just pull an old Panaflow out of an old medium duty AT power supply" They said I was just being cheap. Nope, these are expensive fans, they were hard to find back then.
I have a buch of these things, all pulls from old workstations, etc. They are quiet and have excellent flow. Think about it, workstations put out a lot of heat, but manufacturers wanted them to be quiet for their customers.
So you probably don't have anything that old sitting around and will have to buy new. That's fine, they are available in two speeds. The low speed ones are good enough for most systems. The high speed ones still aren't very loud.
What's the frequency, Kenneth?