Installed 2 new Kohler elongated toilets about 2 years ago. They both began to
make sucking sounds when flushed, & the sounds have increased in number –
like 4 or 5 times they are ‘gasping’ for air. A local plumber seems to think there’s
a blockage in the drain to the street, & suggested inserting a camera to inspect the
interior, but I don’t follow that logic. It seems to me like the more likely problem
would be a blockage in the vent.
Your opinion?

If there's a blockage, it's in the drain.

I think you should go with the plumbing instructor idea. Checking out inside part of the toilet using video camera will help you find out exactly what's happening inside.

Plumber due tomorrow - will report findings.

Regarding the vent, if a bird craps on your vent opening on your roof enough times, or builds a next on top of the vent, you could hava a vent that is choking for air and causing girgling at the toilets because they require a lot more vent air that the other fixtures in your home. Good luck.

Plumber snaked toilet & cleared the clog, then opened clean-out in from of house, & roto-rooted drain in both directions - to street, then back up to house. Advised water was not running swiftly enuf from house to street sewer, so drain 'slope' may not be steep enuf. Also said don't use double thickness toilet paper - or Charmin (said 'Charmin' toilet tissue is 'plumber's best friend). Both toilets still 'suck air' when flushed, which is annoying, but seem to work.

Comments?