Downstairs room tenant was out living in hotel for last 4-5 days as noise of these machines is too much. 

But IMO if 40% of rent was not being paid widely there would be a lot more noise 

The reason is because if you have an apartment building with multiple tenants, it's easier or more accepted that when you're a tenant in that building, you expect to have the noises that are normal for living in a multi-family building and are prepared to kind of blend in - it's more anonymous.In a duplex, it's more like having to get along with a roommate that you can't kick out.  

The place smelled of pizza & the noise & heat from below was terrible. 

Ive been doing my dueldillengence and trying to studying up on market trends, but everyone is saying different things and it hard to block out the noise.

I don't know, I look at these places and don't get me wrong they're nice, but 300K seems just too much for the square footage, hence I can hear the bubbling noise ;)

Prefer someone with violent criminal history & as many noise complaints as possible. 

There seems to be a lot of people against them, mostly because of parking and noise issues.

Do any of you have ideas as to how I can buffer the noise in those vents?

I also charge a $300 fee up front for each cat or for any dog that has been owned for less than a year (puppy or rescue).Finally, I have an animal addendum that stipulates the rules about being leashed, cleaning up after, noise, damages, and more.There's more to it but it's definitely worth investigating and figuring out how to reduce risk while increasing profit.