The washing machine is in the kitchen, about 15' from the kitchen sink. We replaced the double sink (with garbage disposal) with a single large sink (no disposal). The sink and washing machine each have a vent off the traps. The sink drain used to meet the washing machine drain directly below in the basement, then they traveled approx. 30' to the main drain.
Since we replaced the sink, when the washing machine does its first spin cycle (and only the first spin cycle), the water in the sink trap gurgles and bubbles. So, I split the drain connection in the basement, and ran a separate pipe from the sink drain about 25' further down, where it again meets with the washing machine drain. I replaced the washing machine drain pipes that were 1 1/2" with 2" pipes. I cut out and cleaned approx. 25' of washing machine drain (no clogs).
I'm guessing that the washing machine is creating a vacuum that sucks at the sink trap. Other than running a complete separate line for each to the main drain, is there any way to keep the washing machine from creating that vacuum that backs up close to 30'?