This house is on a slab with two bathrooms . When we use the washing machine the water level in both toilets drop an we hear a gurgling sound .Nothing is backing up. We also have checked the yard where the field lines are located and didn't notice any drainage are smell. Please give us some idea as to what it might be. Thanks Tom.

Thomas and Caro: You have attached your original question to a 10 year old post. I have moved your post to make it a new question.

To answer your question:

Water level drops because you may have clogged vent, or your washing machine is not vented properly. Consequently, it is grasping for air during water discharge, making water level in toilets to drop

If clogged vent, then have it snaked from the roof. If in need of an air vent, then I suggest you add Air Admittance Vent someplace at the washing machine drain location.

Hope that helps

Milo

I agree with Milo, you have the symptoms of blockage in the air vents, or main sewer line, and they should inspected to make sure both systems are clear. How long has this been going on, or has it always been this way? Did you just add the washing machine to the system?

http://homeguides.sfgate.com/causes-toilets-gurgle-washing-clothes-69542.htmlSometimes

You cannot always tell if the main drain is running slow until the blockage is great enough to overflow visibly somewhere. Please let us know what you find.