A draft inducer or vent blower is the most common problem on a gas furnace or boiler to cause a noticeable noise.
Furnace draft inducer blower noise.
Do you hear the motor whining or some other problem.
Once the vent is moved through the pressure switch the system comes on and starts heating.
A furnace inducer motor noise can be one of the main problems home owners experience with this component which is mostly found in newer high efficiency furnaces.
You can fuse the furnace model number to shop around for the inducer motor assembly and probably buy it for 200 250 or so.
Usually the inducer motor assembly can be removed and replaced in a half hour or so.
Furnace draft inducer blowers push air and gases out of your furnace and towards outside improving the quality of the air that moves through your furnace.
My fasco furnace draft inducer blower has become noisy rattling rattle clicking sound.
Also what are you noticing.