It may be that noise transfers easily through the units.

We don't allow dogs in our upstairs apartment above our commercial building because the flooring is hardwood and it causes way too much noise.

Keep giving them 3 day notices to quit making noise or whatever else you can think of that would be legal and related to what they're doing and that violates your lease. 

My signal-to-noise ratio coming from Zillow et al. is not good.  

You have to follow certain code requirements, such as :you can't house more than five unrelated individuals at a timeyou have to abide by all noise ordinances and parking ordinancesyou cannot serve breakfast or else you fall under the B&B lawsTo the question I would have and I imagine others have, Can you buy a property down there and rent it full-time?

How about when family comes to stay for two weeks and you get complaints about noise? 

The standard lease basically says, “Don’t make noise and be a nuisance to the neighbors.”

I wouldn't call losing 1/3rd of your living space, having strangers making a lot of noise in your home at random hours, having to take off work to supervise those strangers to make sure they don't steal anything, and having an area unsafe for kids when you have kids an inconvenience.If you aren't providing a habitable unit the tenants are not required to pay rent regardless of if you say so in your lease.

There could be some pluses - like less noise. 

But more importantly they'll eliminate most of the noise that can be overwhelming OOS.