As to the private road issue, the police are correct. There is nothing they can do. If it were a public right of way, it would be within their jurisdiction. As it is, it is a civil matter of the tennants interfering with your rightful enjoyment of the roadway easement.

Some noise ordinances make reference to decibel levels, some are more general and subjective. I have not heard of one with a distance to source provision. Check the local ordinance for yourself to see just what it says. These can usually be found somewhere on the City's or County's (for unincorporated areas) website. If it provides for an objective standard like certain allowable decibel levels at certain times of day, start making a log. Get an inexpensive decibel meter at some place like Radio Shack, make some measurements when the neighbors make noise, and note time and decibel level in your log. After a while your log will show patterns of excessive noise and you can take that to the police as something documented that they can act on.

If your neighbors are threatening you, you family or guests, your pets or property, this is a different matter altogether and one that the police are usually better trained to deal with. Document each such occurrence in detail. The police may not be able to do much about a single minor occurrence, but again, if you have a log demonstrating a pattern of abusiveness, it gives them something more to act on.

Until the tennants' actions reach the level of misdemeanor crime, the police are going to be quite limited as to what they can do with problems which happen entirely within private property. Those matters are often more effectively handled through civil action. Up until you decide to take that course of action, continue directing complaints to the landlord even if they bring no action at all. That way, if you do bring civil action against the tennants, you may be able to pull th elandlord in as a defendant as well. If nothing else, that will motivate the landlord to do something about problem tennants.