Plumbing roof vents, modern vs. old style

[bucolic_frolic]

I'm not finding reference to this issue online, or maybe I'm not using suitable search terms.

Nowadays, from say the 1980s to now, plumbing vents are designed to run from basement all the way to the top of the system - the traditional roof vent.

In the 70s, that might not be the case. Vents would pass code, if there was a code at all, by simply tapping a trap off the highest large point in the drain system - the toilet drain - and going upward through the roof to make the roof vent.

I have such a vent and get persistent gurgling in the downstairs bathroom sink. The water goes down well, but echoes for several seconds with gurgles, and then again. Unclogging the pipe hasn't helped much, though water flow is far better.

Anyone understand these old installs? Is my problem in the roof trap, or in the sink drain pipe? I've plumbed the drain, but there seems no clog, or maybe I'm not into it deep enough.

TYIA