wrote:

Whistle???????

The whistle is so the guy who is filling the tank gets a warning
before it overflows.

I have lived in several houses with oil heat, and never heard a
whistle...... I just had a fill guage in the tank center, and a 2
inch fill pipe, and 1 inch vent pipe. I was in WI.
Is this something local to your area? What state?

The original poster was posting from Canada-- but I'm from NY & it has
been required here for at least 30 years.

They appear to be required in Wisconsin these days --
http://www.commerce.state.wi.us/ER/ER-BST-ResTk.html

They are usually part of the vent pipe &unless you're right there and
listening for it, you might never associate the whistling with the
vent.

Jim