lol, if she's older like you say, then keep laughing and giggling everytime she hums and eventually when she asks why you keep laughing say "my mum hums all the time, must be an old woman thing"
if that fails ...
say every single time "I name that tune in one"
if that fails ...
revert to the AK47 idea
PMSL How very cutting..polishing up ak as we speak .........