We're in the process of adding a second bath to the basement. We had the concrete broken and the soil pipe tapped into to run a new line to where the new bath will be. This was all done by a pro.

Shortly after this job was finished (note that we have all the connections 'capped' off in the basement until we can finish the rest of the bathroom - so no fixtures yet) we noticed that after flushing the toilet on the second floor (existing) we would hear a bang or thud coming from we think the floor area (or somewhere below us).

Any one have any idea what it could be?