When the air is trapped at the highest point, which is the core, you get this noise. Universal problem across all manufacturers, which is why the tool was made.

If you have a core leak you also have similar noise from water moving around in the box and from the air introduced into the core from the leak.

Use the tool and you will quickly see you have a system leak. When you go to pull vacuum you will see the vacuum won't hold.
Up to you to figure out where the leak is.