I have a Coleman 7670 gas furnace, originally installed in a 1973 mobile home. Once the thermostat kicks in, there is a rumbling noise as it is warming up, but goes away once the blower starts up.

Is this a serious problem or an easy fix? I'm in the process of selling the house and don't want to invest any more into it than I need to.

Thanks!

Could be the gas pressure is to high. Could be a problem with the gas valve especially if it is a step valve that is used in Mobile homes. Could be a dirty burner assembly. I have no idea how serious this is but it is not normal. I would have it checked out.

I agree with hvac100. It sounds like maybe it's burning dirty or too high of gas pressure. Is this LP or natural gas? You may be able to take this apart and clean the burner and orfice yourself but to test the gas pressure you will need a meter. You may need to call in a pro for this one.