DIAGNOSIS This is almost always the convection fan motor or its bearings failing. The motor is located in the rear cavity behind the microwave.
FIX Unplug the microwave. Remove the outer casing. Locate the convection fan motor (attached to the metal air guide). Check for free spin. If it's grinding or seized, the motor assembly must be replaced. Ensure the fan blade isn't hitting anything.
IF THAT DIDN'T WORK A failing magnetron can sometimes buzz, but that's rare during convection-only cycles. If the noise is only during microwave operation, suspect the magnetron or high voltage diode.
HEADS UP The convection fan motor may share a mounting plate with the magnetron cooling fan. Label wires and take pictures before disassembly to ensure correct reassembly.