I have been in this house for 6+ years and have recently started having this problem. No changes in plumbing or appliances have been made. When my washing machine discharges it is bubbling the soapy water back into my toilet causing it to overflow and is also backing up a bit in the sink but not overflowing there. The washer,toilet, and sink are all located on the bottom floor and in the same 1/2 bathroom. I also am on a septic system. As I said the water that is backing up is soapy and does not appear to be sewage just soapy water. When the washer is not running the toilet and sink seem to work fine. The rest of the plumbing in the house on the 2nd floor sinks, showers and other toilets do not seem to have any problems. Is this more likely a problem with a clog from the 1/2 bath drain line feeding the main drain line, maybe a clogged vent or a clog in the main line drain to the septic? Where should I begin? I want to hopefully take care of this problem and hope to avoid a much bigger problem. Any suggestions would be appreciated. I have listed what I think could be possible problems but would like to get pointed in the right direction from a professional. Thank You.