Hey everyone; new user to the site and was hoping someone may have some ideas for us!

My wife and I just moved into a home that was built in 1987 that is connected to city water and sewer, but in the past had well and septic. We had a renovation done in our kitchen that put new granite with double sinks and a new disposal (there was no disposal before). Everything worked fine for about a week. On thanksgiving, both of the sinks wouldn't drain and backed up. I took apart everything underneath the sink and checked the traps and the pipes. There was no blockages. I put everything back together, strangely enough everything worked again. We ran the dishwasher with no problems etc. Then, the next morning it happened again while my wife was doing dishes. I removed everything again and ran a 15' snake down the pipe that the sinks connect to. Nothing blocked. Now it's still backed up. Also, the disposal has a dripping leak from the bottom of the housing. I also heard some sort of alarm which sounded like it was in the pipes somewhere when I tried sink this morning.

The two sinks drain into one 1.5" pipe that is by itself. The 1.5" pipe runs through the basement rafters and connects to the main line which is exited into the sewer. There are no other problems with any other sinks / drains / toilets etc. I'm about to start beating my head against the wall on this problem. Any help or suggestions would be GREATLY appreciated!!