Banging noise from toilet after flushing

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?

Have someone flush while standing by the stack in the basement..
See if you can locate the exact source of the sound...
Might be so simple as to be a loose pipe....

When the new plumbing was stubbed (connected) into the existing house water supply, being that the new work was in the lowest part of the house, it necessitated draining at least part of the system. My guess is that prior to the work, some location in the system had a column of trapped air which absorbed the shocks associated with water flow (as in a toilet filling) when it was suddenly shut off, as happens when the toilet tank reaches full, especially with Fluidmaster sudden off type Fill valves.

If my guess is correct, having a water hammer preventer added to the system should cure the problem.

Not an Expert.