Water Hammer Just Started

I live in a triple-decker condo built around 1909. The third floor condo just replaced their entire bathroom. While it may just be a coincidence, the pipes feeding their unit have had a LOT of water hammering since the work was done. This is especially true with the hot water where the noise is especially loud at the water heater (I live on the first floor and am the one who gets the brunt of the noise at 5 a.m.!!). Any thoughts on how to fix this? And why would it start just after the construction? Thanks...

Talk to the people on the third floor and ask them to check it out.

Could be just a loose washer in a shut-off valve associated with their unit.

This is just puzzling us... The plumber who did the work is talking about a need for water hammer arrestors. This seems completely unnecessary since there was NO problem before the work was done. Also, the shut-off valves + the water heater are in the basement (the bath renovation was on the 3rd floor). No work was done on the shut-off valve or any plumbing in the basement.

Thoughts on what on the 3rd floor would have caused this change?

You may have a valve that chatters and instead of an air hammer.

A chatter is when the washer in the valve is loose or is warn thin around the edges.

There may be supply stops at the fixtures in your bathroom that the contractor used . Try closing those stops and then open them up all the way.