Sound of allergic reaction from one half of train (5)

'sound of allergic reaction' is the definition.
I know nothing about this answer so I can't judge whether this works.

'one half of train' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'half of' suggests halving the letters.
'train' becomes 'choo-choo' (childish name for a train).
'choochoo' halved is 'choo'.
'a'+'choo'='ACHOO'

'from' is the link.

Can you help me to learn more?