I live in an apartment complex where plumbing work was recently done right outside my building's window. Now, I'm hearing a whistling noise, sort of like a tea kettle, it doesn't stop unless you run the water on a faucet. The complex has called the plumbing company back out to troubleshoot but it seems no one can figure it out. It's impacting me and the tenant right under me. What could this be? It's driving us crazy. It seems to get worse at night.

What work was done? A bad pressure regulator along with a slow leak could cause the noise. A problem like this usually requires onsite troubleshooting.

I'm not sure,but it was extensive. They were working on pipes underground and they also opened the wall right outside my kitchen. It sounds like the noise is coming from that area. They worked on it for at least a week. The noise came shortly thereafter.

Will the noise go away with anyone else(besides you and downstairs neighbor) using water?

Yes, it seems that way. I believe it's worse at night because most people are asleep. Seems like the noiss is constant between midnight and early, early morning. When I hear the upstairs neighbors running water, I don't hear the noise.

i would start with all the toilet fill valves.
possible debris in one making it run and whistle ?