Dirty water coming out of the faucet only at nite?

okay i work for a small restaurant in a town and only at night dirty water will come out of every faucet the kitchen,woman's bathroom,Men bathroom,and even the soda fountain, oh and when it first starts to happen when you turn on the water its like theres an air bubble in your way...does anyone know what could be causing this cause like I said it only happens at nite..thanks..