It's not the noise, the problem is that since they are in series, they will draw the same current, and split the voltage in half - until one of them stops. When that happens, one of those fans will start trying to draw more current - or equivalently decrease it's impedance, making the voltage divider not so equal - and probably exceeding one of the fans' voltage rating and releasing the magic smoke.
Worst case, this will happen when power is first applied, and one fan decides to wait a fraction of a second before starting. Zeners would limit the voltage to the fans to something reasonable, and allow it to startup properly.
Like I said - paranoid. It'll probably work without it, but I wouldn't want to put money on it , especially if there is a chance of something halting one of the fans for any reasonable (couple seconds) period of time.