DIAGNOSIS A new rattling noise from the vent fan almost always means something is interfering with the fan blade. Often, it's a loose piece of the charcoal filter that broke off and fell in, or the fan blade itself has come loose on its shaft.
FIX 1) Unplug the microwave. 2) Remove both grease filters. 3) Look up into the vent opening with a flashlight. You might see the debris right away. 4) Use long-nose pliers or a magnetic pickup tool to carefully remove any foreign object. 5) Reach in and try to wiggle the fan blade. It should be tight. If it's loose, you'll likely need to remove the outer cover to access the set screw on the motor shaft to tighten it.
IF THAT DIDN'T WORK If the blade is tight and there's no debris, the motor bearings themselves might be wearing out, causing a grinding rattle. In that case, the whole blower motor assembly needs to be replaced.
HEADS UP Never run the vent fan for extended periods with this rattling noise. A loose object can damage the fan blade or motor, and a failing bearing can seize up, potentially causing the motor to overheat.