Okay, so here is the deal. We have two separate full bathrooms on different lines. The master bath has a full shower, toilet and sink. Lately we have noticed that our tank in the toilet will randomly fill with water for 1-2 seconds and bubbles come up from the bottom of the tank inlet tube. There seems to be no pattern on when this happens; after flushing, time intervals or so forth. However, we have noticed that if we turn the sink or shower in that bathroom on the same action happens when they are turned off. I also can hear a gurgle (not plural) in the wall pipe downstairs below the bathroom. Again, seems random.

Also, when the sink or bathtub is turned off and it doesn't perform this action, the ball float vibrates, sometimes enough to let water out of the tank.

Finally, we have a home built in 2003 with no fancy piping under the sink and such.

Thanks for the help!