Folks, need help here.

I have top floor condo rented out in last one year. The downstairs neighbor has complained about squeaky sound and heavy foot steps constantly in last ten months. I have worked cooperatively with him and the tenants in the last ten months trying to reduce the noise. I even fasten the sub floor with screws and changed the floor in bathroom to reduce the noise. But this guy keeps complaining the squeaky sound from the bathroom area.

Some background about the building and the rental: it is a thirty-year old four story building with wood frames. The previous owner is a pair of old folks living there for last twenty years and I bought it last year and two sisters in their 30-40s moved in. My guess the old owners don't walk lightly due to their illness and the new tenants are more active. And it is a old building and squeaky sound from the joists are expected.

Now I received a letter from HOA about the noise. To make things worse, the downstairs neighbor also sits on the board. Any suggestions what I should do here to work with the board and also protect my tenants and my rights? Thanks!