I would like some troubleshooting suggestions for my problem with furnace.It won't turn on,the thermostat is an old mercury type.I can hear a clunk type noise when I reset circuit breaker.How can I isolate the problem ? Thermostat or some other component?

Are you saying that the furnace is tripping the circuit breaker?

I only flipped circuit breaker in an attempt to reset unit

I only flipped circuit breaker in an attempt to reset unit

It did NOT trip the circuit breaker

If you are hearing a clunk noise, it almost has to be associated with the fan and or motor. Open unit and see if fan and motor will turn by hand.