HVAC guru's

Here's the setup: Residential new construction

Problem: Detected ununsual 'poping' noise and air rushing from the bathroom sink drain. When I fill the sink with water, it's slow draining and air bubbles come out.

Scenario: The primary HVAC drain is tied directly to this drain under the sink, before the trap. There's no trap at the air handler in the attic. The builder says the trap at the sink is fine. However, most new construction I'll typically see a trap close to the air-handler in the attic. Client is complaining with the effect of the noise and air rushing from the drain.

Possible solution: By installing a trap at the air-handler in the attic, would that solve the problem? Or is there another trail I should go down?

Thanks in advanced for your help?

Regards,
Phil