Pipes knocking if sink turned on while shower is running

Hello, I'm hoping someone can help me identify this pipe in a house we recently purchased. It's an iron pipe (~1-1/2" I think) that runs from one of the sump/seepage pits in the basement up into the attic. It appears to enter the side of the pit just below the floor. Goes up the basement wall and into the exterior wall. It comes back out of the wall near the ceiling of the main floor, above the kitchen cabinets. Then disappears into the attic. I haven't been up in the attic yet to see if I can find it as it's been so hot. I recently discovered water on the counter, running down the cabinets under the pipe. So I might have to go into the attic anyways... House is in the western Chicago suburbs. Built in the 50's. Thanks!

There never was actual water coming out from beneath the toilet but it always to seem to be glisteningly wet near the base and with the door closed for a while you could detect a slight sewery smell, so I yanked it. First thing that I noticed was that there was some sort of raised seal gunk underneath which shattered. Not sure what it was. Second thing was that I thought there was supposed to be a little lip to the drain pipe but this was cut to the level of the tile. So I plopped the new seal down. Placed the toilet on it. Tightened the t bolts and flushed and water came out the sides. Real water, not just the moistness of before. What did I do wrong? Here's a pic: