There is a wide spread phenomenon of pre-velar nasal tensing (in general, pre-nasal tensing) in AmE: CAT vowel becoming BET vowel.
an apple: ɛn'æpəl (pre-nasal tensing)
The official version according to most sources is:
/ən; when stressed æn/
However, if you use Google dictionary to listen to the 'an' it sounds like an ɛ.