We have a house built in the late '20s or '30s in a place that may not have any real building codes.

Our shower, bathroom sink, and toilet all drain into the soil stack, (via a drum trap), which exits through the basement wall to the septic system.

Our kitchen sink and washer drain down through a separate drain, through a pipe running out the basement wall on the opposite side of the house from the soil stack. The kitchen/washer pipes do not appear connected to the septic pipes at all.

Our basement has three floor drains. The drain closest to the sink/washer appears to lead towards the area of the house with the soil stack (and the other two drains are closer to that side of the house).

Our gutters may drain into something connected with the floor drains, as sometimes when draining the kitchen sink or doing laundry, if we go outside, we can hear a gurgling or bubbling sound from the gutter downspout. Weird!

Sometimes after taking a shower, or doing dishes, a nasty smell comes up from the basement floor drain. Even though I wouldn't think the two (septic and kitchen/gutter) systems wouldn't be connected.

A neighbor suggested there might be a clogged "grease trap" which might account for some of the weirdness, but we have not been able to locate any such trap.

Does anybody have any idea why these things might be happening? The plumbing just seems so strange and not like what I've read about in books, even books on old houses.