For a couple of months now the downstairs toilet has produced a squawky/farty noise a second after being flushed. It seems to come more from the wall next to it than the actual cistern, and it can actually sound louder in other rooms of the house. It doesn't do it every single time, and varies from a tiny squeak to a heart-attack inducing howl. Being a little short of money at the time, we have mostly just used the one upstairs for a while. Yesterday the upstairs one joined in ever so slightly, which is worrying. Could it be air in the pipes, and is there a DIY way to get rid of it? The house is only 5 years old, and no work was carried out before this started.