First, you should speak to each tenant to find out for sure what is going on. Ask them to write down all of their complaints with specific details such as the dates and times of all the noise incidents. If you believe one tenant is causing the most problems then you may have to serve that tenant with a Form N5 detailing all the issues. The N5 is a 20-day notice in which they have the first seven days to correct the situation. If it is not corrected after seven days, then you would file an L2 application with the Landlord and Tenant Board.  In some cases where both tenants are equally disruptive the N5 may have to be served to both tenants.