Found the house with the bunker underneath it, with the access in the waterwell.
Set up shop there a bit to loot the houses in the neighborhood... at night I was placing some torches around the place (hate the strange lighting on those... great light when u hold it, but when u place it, it gives the light of a small candle irl)... anyway, doing that somehow made zombies above attack the base.
It's just ridiculous how 'smart' and good at hearing the zombies now are... you can't do anything without them hearing you I guess... even if you are deep underground surrounded with concrete.