Furnace makes noise before giving heat?

Hi Booby, The noise could be coming from a loose blade on the squirrel cage (the fan blade assembly) If it gets loose and rattles or even comes apart from the rims, the blade can tear up the housing or worse catch on the housing and burn out the fan motor. Have someone check it out quickly so that you don't incur more expenses by damaging other parts. Expect to pay between $150-$300 for the fan blade replacement and labor. If you wait too long and the motor burns out, you can double that. https://s.yimg.com/hd/answers/i/89e205094bdd490592e0a4f365fe2bf0_A.jpeg?a=answers&mr=0&x=1402673957&s=f1fcd4fb8d408cff58bf69d80ff4ba7f

Maybe you ARE hearing something spinning at high speed. A fan, which your furnace must have if it's a forced air furnace that blows heat out the vents. The fan is what blows the air. You didn't say if this is a new noise or not. If it is, you may want to call someone to have a look and make sure the furnace doesn't need a repair. Doing that is better than having it break down suddenly.

The thermostat on the wall starts the burner. As the burner works & starts heating the furnance, there is a second heat sensor inside the furnance, feeling how hot the furnance is getting. When this second sensor feels the furnance is now hot enough, it trips & turns on the blower motor that pushes the air thru the hot furnance & into the house. You are probably hearing the blower motor turn on. Normal.