We recently had the septic tank pumped; we've owned the house now for 2 years and it was built in 1965. Right before we had the septic tank pumped, we noticed that sometimes when the washing machine was at the spin/rinse cycle and one of the showers was running, the toilets would burble and water would flow up from the bottom of the toilet in the guest bathroom at the floor, (not an overflowing bowl); it's the farthest from the washing machine. The toilet bowl in the master bedroom (closest to the washing machine) would raise the water level, then drain down - would not overflow. Pumping was about 6 months ago and we've just noticed this happening again. It's clean water, not like it's coming out of the septic tank. Does anyone have any thoughts so I know where to start?

If it is clean water, you are most likely getting a backup on the drain side of the septic tank. Have a plumber snake out all the drains to the septic tank.

I agree but also think that bath with the water on the floor also needs a new wax ring. A back up can raise a toilet bowl level, flood into a tub but should npot be on the floor with no overflow.