What is the model number of the microwave oven.
A loud electric type sound on starting in humid conditions may be an indication of sparking occurring between components or components and the cover of the microwave oven when there shouldn’t be.
The microwave oven sitting above the stove will get all the steam (particles in the steam) etc passing though it and this may leave a coating on the components. In humid conditions, current can flow because moisture in the accumulated coating conducts electricity, especially the high voltages used in the magnetron circuit i.e. HV diode, HV capacitor, HV transformer etc.
The oven would need to be opened and the components checked to see if they are clean from any coating caused by the steam etc. Look for any obvious signs of sparking e.g. burn marks on the inside of the cover or on the board.
Be safety aware when working in microwave ovens. The HV capacitor can store >5000V DC for months even if the power has been disconnected of this length of time. This amount of voltage can seriously injure you. The HV capacitor needs to be discharged correctly as soon as it can be accessed after removing the microwave cover, before commencing any other work inside the cabinet. If you don’t know what you’re doing then don’t do it! Call a reputable, professional appliance repair service and ask for a quote.