What could be causing a rumbling noise in our house?
I recently experienced the same issue.  We just found out what it was and had it fixed.  Apparently, there is a pipe that sticks out of the house from the HVAC system underneath the house that releases water that is formed during the cooling and evaporation process.  We found out we needed to have the trap (device that collects debris) checked and cleaned annually.  Because the trap was full of debris, water couldn’t leak out of the HVAC system and out the pipe causing the build up of pressure in the HVAC system (cause of the rumbling noise underneath the house).  Eventually what happened was, our central air unit had pressure problem and could barely pumped out any AC.  Once we had the trap cleaned, ever thing started to work again and the noise went away.