First-time vapers often think a whistling or rattling sound means something is wrong. But airflow noise is normal, especially with tighter draws. It's the sound of air rushing through a narrow channel.

That said, some noise can indicate a problem. If you hear a gurgling sound, that often means the coil is flooded—too much e-liquid in the wick. If you hear a hissing sound after you stop inhaling, that could be the coil still heating up, which is normal.

When you're testing a device for your store, listen for these sounds. A little whistle is fine. A loud, constant gurgle might be a sign of a design flaw that will frustrate customers.