Bad smell from basement!

[JessicaT]

My house is on a block foundation. It needs to be dug around outside and resealed. Estimate was over $30k so that is not an option. When the block fills with water it drains to a sub pump that goes to the septic tank. This was filling the tank so we rerouted to another tank that pumps to the ditch. The issue is the smell in the basement when it rains and now is starting to flow upstairs! Sometimes it smells when it is too dry also. Not really a sewage smell but sour. We have had 4 plumbers here. One sealed a vent but that didn’t help. There is a cystern, I am pretty sure it is sealed off because we can’t find a way in. The lid is under the floor and I am not really sure about taring that up to get to it. Any ideas at all what the smell could be? I need help!!!