1. Contacting police when their behavior after hours wakes you (as you describe the screaming at 3:00 AM). It creates some amount of evidence you may use later, it puts LL on further notice of the situation in which you.
2. If neighbors threaten you in any way--contact police and let the LL know as well (part of the letter addressed at # 4 below.
3. Document in contemporaneous notes, each incident, time, date, what happened--keep a running log.
4. Sent written letter to the LL demanding he take action re the cars and the noise. Remind LL that you have a right to quite enjoyment of your leased property and his failure to ensure your right is adequately enforce may result in you holding LL in breach of contract and termination your lease agreement, suing for rent abatement, attorney fees and damages associated with the early termination. Send the letter certified mail, return receipt requested. Your LL is NOT powerless and their presence for all those years does NOT entitle them to rights or privileges such as you have described.
5. Recommend you chat with a PA landlord tenant attorney. That attorney will know about local ordinances and state law, he/she will know the local court leanings and may very likely know your neighbors since they've been tenants for so long. That attorney will be able to identify if you can recover attorney fees and what other steps you can take (calling code enforcement, setting up escrow account with the court etc.).
6. Lastly, understand that apartment/duplex living has its challenges; total silence can't be achieved and sometimes it will come down to the degree of discomfort the neighbors create for you and how far you are willing to go in order to change the status quo (terminate and move, sue LL, etc.).
READ THIS BEFORE CALLING OR EMAILING ME: I am licensed to practice before the state and federal courts in Virginia. Addressing your issue does not create an attorney-client relationship and I AM NOT providing you legal advice. You should speak with an attorney to whom you have provided all the facts, before you take steps that may impact your legal rights. I am not obligated to answer subsequent emails or phone calls unless you have hired me. I wish you the best of luck with your situation.Ask a similar question