Three years ago I bought a brick row house in Washington DC, built in 1893. In May 2009 I had the front (north) wall repointed and repainted. In June, I began to notice a musty smell near the north wall on the top floor, then also on the floor below. There is no visible sign of water damage. I called in my roofer and plumber and the mason, and then two industrial hygienists. The roofer, plumber and mason found no problems. The hygienists confirmed there is water inside the walls in various places (behind both drywall and plaster), but they could not locate the source. All the contractors agreed that a pipe leak should be causing visible damage by now. My neighbors with similar houses have no problems, and the previous owner denies any knowledge. The musty smell is much less pronounced now. Clearly I have a problem, but I'm reluctant to start demolishing the walls without some idea of what I'm looking for.
Could rainwater be migrating through the new exterior mortar and/or paint? Any other ideas? I'm at my wit's end.