Some very insightful info guys, thank you!
As a matter of fact there is a u-tube manometer connected to the exhaust pipe that I can monitor in the garage.
When the unit is burping, you can watch the red liquid inside fluctuate in level...
And I believe someone asked how often the fan runs: it runs constantly. The on/off switch was installed in the attic of the garage to make sure it was never accidentally turned off. (Amazingly, this was covered by code on the installation of radon evacuation units!)
I'm tempted to try the small hole in the lid, but I'd hate to find out I'd created a 'whistle'... That would be going from annoying to awful.
Chet