If both parties are your tenants, it is still best to leave them to resolve their dispute on their own. Include a clause in your agreement that requires tenants to sort out their disagreements amongst themselves to the best of their abilities. If you must intervene, make sure your agreement reflects that tenants may not always like the resolution. When getting involved, talk to the noisy tenant in a calm yet stern manner. Let them know that someone has complained about the noise and ask them to avoid making noise in the future. It is equally important that your lease agreement restricts the amount of noise allowed.