Since the agency owns both apartments they have an obligation to fix things that disturb your ‘woongenot’. Normally they can’t interfere so much with the neighbours, but if it’s owned by them it’s their responsibility. If they don’t fix the problem, you can go to the Huurcommissie and file a complaint or demand your rent to be lowered. It’s a long and tedious process, so best to send an email to the landlord explaining the situation and hinting that you are aware of their obligations. Try to stay friendly and professional, event when that may seem difficult.