Did this air?

Having air trapped in the pipe might also cause the noise but that usually includes frothy water as well.I have not heard of the copper pipe making noise just because it's not properly connected but it's possible. 

No central air...has window units.  

Yep they didn't have a dishwasher and no central air.  

Landlords almost always try to replace one industry tenant with another so the possible competition should be a potential concern to you.Con : You may have to become a handyman or deal with various ownership issues like heating, ventilation, and air conditioning (HVAC) units, building maintenance, etc., that otherwise a landlord would have taken care of for you.

How do You know if your tenants are changing the air filters?

People say they will sue everyday and it means nothing as they know 99% of the time it is hot air and people will not spend the money or time.

When I advertise my deals that are fix and flip I say:-Kitchen is 10 years old-Bathroom is 5 years old-Furnace is gas, forced hot air, and 25 years old-Roof is 7 years oldEtc Etc Etc...  

I'm an Air Force guy myself and have been investing in real estate for the better part of the last year.

And then there are also vents for air intake such as this link that might possibly allow snow to get in:http://www.dciproducts.com/html/smartvent.htm