Four of our five units have an upstairs and a basement. Central air systems are used for HVAC. The potential for noise problems is extremely high, especially in bedrooms as air supply vents are within a foot or two when a bedroom is "on top of" or directly above another bedroom.
I purchased a Cambridge QT100 sound masking system, quite expensive, as well as 4 speakers, quite inexpensive, and had a fried who installs home audio systems help me run the CAT5 wires for the speakers between the floors.
That was a few years ago... I've pondered this situation since then and think I've come up with a more cost effective solution using a Raspberry Pi, a stereo receiver, 4 flush mount ceiling speakers and an mp3 audio file. I captured the audio file using a box fan from Walmart and an mp3 recorder, a Sony ICD-SX712.
From my experience, white noise devices are sorely inadequate at masking noise when vacation rental guests complain of noise outside or in adjacent units.
A relatively inexpensive solution is to play a white noise audio file on a laptop with speakers or on a bluetooth speaker.
For those looking for a bit more, I purchased a Raspberry Pi, a vintage analog stereo receiver, and a couple ceiling mount speakers. The Raspberry Pi plays a white noise mp3 file that goes into the receiver and out the speakers in 2 bedrooms. The white noise masks any outside noise or noise in adjacent living spaces above or below. You can even create your own mp3 file using an mp3 recorder and a box fan