In either bathroom, when we flush the toilet, the tub gurgles, or if we drain a full tub, the toilet gurgles. In the morning(heaviest water use), we are getting stout septic odors in the end of house closest to septic exit and cleanout. I have snaked the vent behind the washing machine, and both vents on roof. I ran a garden hose down roof vents and didn't get any backup into sinks... and watched water flow through cleanout. I've snaked from cleanout up to tee under the house but it seems to flow freely. This is an aeration system on a mfg home. Less than 5yrs old. What other problems could be causing the gurgling and odor??

First off, NEVER SNAKE AGAINST THE FLOW. You have no idea where that snake will come up. If it comes up under a toilet bit could crack the bowl.

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In either bathroom, when we flush the toilet, the tub gurgles,

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And with that on sentence you have analyzed your problem. You have a blocked roof vent. Let me explain,
When you flush or drain the running water sets up a vacume in the system that is relieved by the roof vent. The vent's blocked the suction has to vent itself some where .
The "gurgle" you hear is the system venting through the water in the trap.
The solution would be to snake the vent from the roof.
Good luck, Tomk