Welcome in H.C.M. for consultation.
I have gone through your query regarding Whooshing sounds and your own voice is heard like your head is in a tin can.
You are suffering from one of the complication of grommet insertion. This situation arises because of post operative swelling of middle ear mucosa leading to Eustachian tube obstruction and blood and mucous of middle ear had blocked the tunnel of grommet.
This can be manage by antibiotics, anti inflammatory, decongestant and mucolytic
drugs. this is a common complication and usually resolves well.
Hope your query is explained.