This is an old house with cast iron drain pipes & septic tank...When we turn on the shower the toilet bubbles. Sometimes while in the shower the toilet will flush & the sink gurgles too. I assume it is the vent clogged. Last summer I got up on the roof & blew compressed air down the vent & it came up in the toilet, but the matter persists. After taking a shower the toilet dont want to flush for a few minutes .