I like Speedball's analysis and persistence with correct diagnosis.
Just started having the gurgle in the basement shower, with lower level of bowl water in adjacent toilet.
Speedball says this is usually a vent issue.
How does a vent get clogged?
How do I discover which vent is venting the basement - there are four bathrooms in the house.
Previous owner said he paid $600.00 to have the drains "rotorouted" and another $250 to have the septic tank pumped.
House was built in 1995 by builder/developer and appears to have spared nothing in getting everything done perfectly.
I have a history of fixing a problem that did not exist and creating more work than necessay - the curse of the untrained.
Any thoughts will be appreciated.
Aged Eagle - Peter Neville

Speedball1 is very good and he's persistent. Your vent is open to the environment above your roof. Leave, birds, bird debris and other junk can fall in just by chance. Any of this stuff can cause a partial blockeage of the vent stack. If a draining pipe can flow freely because it can't pull air through the vent it will gurgle. A bit like turning a pop bottle upside down. Check above you roof line for a 4" pipe sticking up. That should be your vent and its common to have only one.

Bal gave you a very good answer. Let me add a bit to it. You ask," How do I discover which vent is venting the basement - there are four bathrooms in the house"
You go to the attic and see if there are any vents that are revented back t6o a outside roof vent. If the plumber revented some of the bathroom vents back to a single roof vent then the vent closest to the basement group will be the one to snake. While this makes for less pipes coming out the roof it presents a hassle for the one attempting to snake a revented pipe. The basement vent will have to be cut out of the roof vent and the basement vent will have to be snaked from the attic. If you only have one or two vent pipes coming out the roof then chances are you're revented in the attic. Good luck, Tom