Pipes sing when upstairs toilet is flushed

[Ula NI]

Hi, the pipes are really loud when I flush the upstairs toilet and seems to be better louder. Could there be air trapped? Any ideas on cause/possible solutions please? Noise in pipes sounds likes it's coming from our boiler room (which is inside the house, off the utility room). Or could it be the toilet cistern, and noise just traveling?
Detached house, in countryside, pressurised heating system (if that's of help,).
Gem

 

[Jurassicspark]

Float valve or washer will need replacing.

 

[Deleted member 267285]

Highly doubt trapped air (a common misconception). As above, check the float operated valve (ballcock) for a compressed washer and might just need replacing. If it’s a new thing, then almost certainly the float valve, if it happens all the time, perhaps some loose pipes.