How to stop flushing noise?

Hi there,

I live in a house, probably about 6/7 years old now with 3 toilets. 2 up , 1 downstairs. (Ideal Standard)

At some point, probably about 3 months ago now, 1 of the upstairs toilets and the one immediately below it started to really buzz after flushing. The sound appears to be coming from air in the flusher mechanism. Sorry I don't know the part names but the soft plastic tubing where the water comes out of and into the cistern is the last element in the chain.. when I fiddle with that, e.g. hold it slightly closed so theres a pressure build up, the noise stops...

It's an awkward problem as we don't like to flush on an evening now as we have a 2 year old who wakes up fairly easily and over time the noise just seems to have got louder.

So any help greatly appreciated.