Hello,
I live in a house which is about 2 years old. We have 5 toilets, and every once in a while, they "burp". There isn't anything coming up any shower drains and the toilets don't seem to be clogged. They flush perfectly fine. I called the plumber who did the original work and he said it was just air in the line, but it kind of freaks me out. We have a well, so we aren't on city water and therefore I know it's not the city flushing the lines. Any help would be greatly appreciated! Thanks in advance--

You have a partial clog in the drainage line. Let me explain what's happening. You flush the upstairs john, The discharge picks up speed and bangs into the partial clog in the line and starts to rebound back pushing a bubble of air ahead of it. This is what you see in the f toilet. Then, because it's not completely blocked the discharge drains away before anything overflows. This can only get worse. The drainage line that services the toilet needs to be snaked out/ Hope this helps and thank you for rating my reply. Tom