Toilet making loud groaning noise when I flush it how can I fix this

Explain "groaning" noise..
Anyway its not "air", your cistern its not sealed otherwise the water wouldn't fall down into the toilet to flush it, think the cistern simply as a bowl of water with a lid for convenience/look/protection.
If the noise is from the water refilling the cistern then it could be the valve in there (NOT the shutoff valve under the toilet), check this https://www.askmehelpdesk.com/plumbi...hed-35192.html it explains it well for you to fix it yourself

What's making the noise? The toilet or the fill tube? Back to you. Tom