I own 2 Estoc Mega
tanks (4ml), 2 Veco tanks (2ml), and 1 Veco Plus tank (4ml), all fitted with EUC ceramic or Kanthal coils.
All of these do not whistle with an airflow control setting of ~50%.
Anyway, these coils ought to be replaced as soon as they start to gurgle, which they do at the end of their lifetime.
But do not worry if they do this after a refill; the gurgling may be gone after 5 draws at most. If not, replace the coil.
Regards
aLyr