jmagnier wrote:

For the past two days, I've been noticing a really loud 'boom' or
'thud' coming from the general area of the furnace. It is a gas
furnace, and it has been extremely cold for the past two weeks. I
think the past two days are probably the first few times it has shut
down and restarted, as it was running pretty constantly to keep up with
the cold before. The thermostat is set at about 68. Is there something
I should do here? What would cause this? It sounds like something that
might occur as it fires up, but it's very alarming!

How long have you lived in this house?

The main metal ducts that go across my basement make loud "booms" as they
heat up and cool off. The ducts expand and go "boom" when the heat turns
on and the ducts are cold in the unheated basement. If the heat is
constantly on, or cycling frequently the ducts don't cool off and
therefore don't go expand and go "boom" when the heat cycles back on.

However, if the cause of the noise is expanding ducts and you've
been in the house for an entire heating season I would have expected you
would have encountered these noises before now.