This one has me baffled... Flush the toilet a few times and pretty soon the P-trap in the shower is gurgling. Clogged vent, right? Went up on the roof and rodded 25' down the vent and pulled back nothing. My SO ran the shower and I could clearly hear the water flowing so I know I'm at the right vent. Flushed the toilet and same thing. Clearly heard the water flowing, no muffled sound at all.
The vents in my house are only 1-1/2" but there's a vent for every fixture, except for two toilet/shower combos. This is a single story house on a slab. 8' ceilings and about a 4/12 pitch so that 25' of rodding at the vent went well into the drain line. I figured (hoped) that I must have pushed the obstruction clear so down I came and did a few flushes but it was hard to know for sure just by the sound (but I was doubtful). By the end of the day there was still no gurgling in the shower so I put the ladder and tools away and cleaned up.
Next day, after many flushes, the gurgling starts again. Shower P-trap has emptied again.
If the vent is clear, why is the P-trap in the shower emptying? This makes no sense.