150 s space spaving skimmer

Can you please send a video so we can hear the noise you are referring to?

Does the white cap for the red pipe have a thin pipe attached to the underside which sticks down into the red pipe?

Are you attaching the flexible line to the pipe? If so, that's the issue. Detach it from the pipe and just push it to the side and then install the cap. I know there is a video out there that says to attach it but that is incorrect. Push the flex airline up so it is to the top of the red pipe but make sure the line is not kinked. Then push the flex line to the side and install the white cap.