Demand call for ringing noise coming out of the vent when the downstairs system turned. Upon arrival the customer showed me the vent where he hears the noise from and turned the system on. There is a ringing noise but I confirmed it to be coming from the return vent, I pushed on certain parts of the vent and the noise will decimate. I informed the customer that the vent is what’s causing the noise not the system itself. I did change the air filter today and inspected the ductwork for dirt buildup. There is a lot of dirt buildup in the system but the customer wants to wait on doing any cleaning at this time. The system is a gas system from 2016 Overall the system is running and cooling at this time and customer did not wish to sign up for our maintenance at this time.