Shower Drain

Wondering if anyone else has this issue:

When using the shower and the kitchen sink, the shower it seems like the pressure in the holding tank builds up preventing the shower to drain. Then all off a sudden the air pressure releases through the shower drain in a big burst air. It isnt just a few bubbles of air it is burst of air which of course spashes the shower water all over. Then it drains no problem.

The tank is was just barely 2/3 full and it has happened twice, both times when using the kitchen sink, possibly just luck it happened that way.

Clearly the tank is not venting correctly in this senario. The vent doesnt seem to be blocked though because i have noticed bubblle from dish soap foam out the opening when washing a lot of dishes.

Seems strange, any ideas what to check or look into before I tear into it?