Yo DaveO, had the same problem not to long ago. New plumbing addidtion, gurguling , and LOUD when we did the wash.
I had a combination problem.
1. Drain field/leach line plugged up. and
2. a vent stack improperly placed.
Remedy was I replaced and installed a new 100 ft leach line and moved the placement of the vent to the correct postion which is after the p-trap
. that was my fault.
The leach line got messed up by solids passing thru the tank system and getting into the leach lines. This is an old house and I have no idea how long it had been since it had been serviced. Also the house set up empty for 5 years before we moved in.
In theory septic tanks should function with out servicing. However with todays cleaning products, etc. The bacteria can get killed off in the tank thus the tank doesn't breakdown the stuff like it's designed to do.
So if the solids ever get into the second chamber, then into the leach lines they can go.
So every 2-3 years have the system serviced.
I now use a packet of ridex every few months just to help it out.
Placement of vents I learned the hard way. I won't make that mistake again. Heck even learned that they can go up into the wall cavities, and not have to pass thru the roof, at times.
Hope this info helps.
MAC