When my washing machine empties into the washtub, the nearby toilet has recently (about the past four - five months) started to have big air bubbles come out of the toilet bowl, making a very loud bwoop bwoop bwoop sound. The bubbles are so large that they actually cause water to splash on the inside of the toilet lid.

The bathroom sink next to the toilet already has one of those "chimney" vents, so I can't see air being trapped in there, but I'm no plumber.

I've already snaked the house's vent PVC, and I've put industrial strength drain opener down all the nearby drains.

I'd appreciate any help the plumbing community could offer.