Completed 1 system inspection. When I arrived there was a buzzing noise coming from the furnace. The transformer was buzzing and was very hot to the touch and system was idle. Changed filter 20x20x1. Blower wheel is dirty. Checked run capacitor. Tested furnace. Heat is operating properly. Took amp draw on inducer motor. Checked dual capacitor and contactor both are good. Checked refrigerant levels they are normal. Condenser coil is dirty and recommend cleaning to prevent part failures. Found microbial growth in supply plenum. Recommend replacing or installing Uv light. Replaced bad transformer and now buzzing is gone unit is operating at this time.