My inside hvac unit believe it or not only rattles at night I know that's crazy. whenever I remove the filter it seems to suck in more air and stopes rattling the moment I place the filter back the rattling begins. Mind you the air filter is a new filter. Crazy thing is that throughout the morning and evening there is no noise or rattling only at night. The rattling sounds so loud that you would think the motor is going to fall off, but again only at night . Has anyone had this kind of problem before?
I don't know why it would be diferent at night unless you have a multi speed fan. Or the house is opened up or closed more. If a direct drive check the motor mount bolts to make they are secure and the squirrel cage to verify the bolts are not loose. If a belt drive the belt may need tighting checking the pulley on the motor and squirrel cage also would be my checks.