Timer hums before start knob turned

When I turn the timer on my dryer, it starts humming before I turn the start button. We replaced the timer several years back, any ideas what might be wrong?

You may have some voltage leaking into the timer. I would take off the connection and check and clean the connectors on both ends with contact cleaner. Check everything for shorting or fraying and if it all checks out I would replace the timer again.