Helicopters and bell like ringing

HI

Just a question for some those who have answer(s) to my question. Why is it that some helicopters – like the Alouette / Chetak have a ringing alarm-clock bell like sound from certain angles of hearing? What causes such a ringing ?

Would appreciate some replies from experienced pilots / engineers of the Alouettes or Chetaks

Cheers
Dandpatta