Severe, recurrent noise complaint

Brooklyn, NY. There is a group of people who do not live in our building and multiple times a week bring their cars directly in front of our entrance and play extremely loud music (loud enough to drown out our TV and make it impossible to make phone calls). They also loiter on the sidewalk, sit around our building's garbage area and stay there, loud music playing all along, for several hours, and recently even brought a BBQ grill to have a cookout right next to our entrance. They essentially use our sidewalk as their personal hangout. I as well as other neighbors have called 311 countless times, to no avail, and I have even called 911 and receive no help. When I have tried to contact the local police precinct directly, I am told to call 311. Even though this is a severe nuisance, the police has refused to intervene. Speaking to them is not an option due to safety concerns, these are "rough" individuals and I would not be comfortable approaching them.