tndave

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 don’t want to flush for a few minutes….

Redwood

It isn't a vent problem...

It is a drain problem!

Either the line to the septic tank is clogged or, the septic tank needs servicing and inspected. It's a 50-50 and the only way to tell is to open up the tank and have a look.

When was the last time the tank was serviced?

tndave

We had it pumped out last January so it's been just over a year. We had to have it pumped exactly 18 months before that, but there were more people living here then. It seems like it always does it, but I'm not positive about that because I'm so use to it. Water don't back up in the shower, it seems to drain good.

joecaption

Blowing air down a vent will do 0 good.
I'd bet that at least 50% of older houses have no traps under the tub or shower, do not have a clean out on the outside of the house, no or at least a really poor vent connection for the sinks and toilets. All of this was added once inside plumbing came into fashion.
That noise your hearing is water being sucked out of the drain traps that's there to stop the sewer gas from getting into the home.
It would be worth the money to have a real plumber come look this over.
PS a pluged up drain does not make noise, just a real mess. What it can do if plugged up in the past is back up and plug up the vents.

Redwood

A plugged up drain can make plenty of noises...
Like bubbling toilets and drains gurgling, sometimes when the line is really full and close to coming out in a tub or, shower and the level goes back down it can make a toilet seem to flush as well.

If you do decide to call a real plumber make sure that he cleans drains too so you at least have a 50/50 shot at having him be able to fix your problem.