Toilet Bowl Losing It's Water

Hi.. First time poster but longtime lurker here, I've gotten great advice by reading and searching the forums!

I had a toilet start acting up today, when it flushes the bowl fills back up with water (I believe a little higher than normal), then right away it starts making a slurping-chugging-water siphoning sound and the water level in the bowl goes all the way down to almost empty (I can see the water bounce a little with the chugging sound). It seems like the tank is working fine, just the bowl is losing its water.

I've searched and have found a lot of different suggestions, but I didn't see another issue specifically talking about hearing the water chugging sound that I hear while it's draining and am hoping that it might point to a specific issue. I'm thinking it might be a toilet clog and am going to pick up a toilet auger, but any advice would be greatly appreciated. I don't see any leaking around the bowl, and the toilet isn't moving at all.

A little more info about the toilet: It's a Kohler, about three years old. Used daily for the first year, then the house was empty for the last two years while up for sale, and we just moved back in two days ago and it's being used daily again.

Thanks!
Michael

After posting this I plunged the toilet and that seemed to resolve the issue. It's flushing normal and the bowl isn't losing water. Now I found that when the shower is running and toilet is flushed the toilet will back up and the problem returns.. sounds like blockage in the drain line or vent right?

It may be a main line. You will want to get that fixed right away.
It can back up onto flooring, carpets, and get the walls wet.

Thanks Terry, I'm getting a pro out to take care of it. The house has been empty with a lock box and people in and out of it for two years, so who knows what's been flushed or dumped down a drain.

Thanks again for the great forums btw!