Sailmonkey is correct. You should never put a vented loop on a suction line.
It is designed to let air into the line when it is under suction. This makes it harder for the pump to pull water in. The OP replaced a partially blocked vent and lost
the ability to draw water into the pump.
If you are worried that water might siphon into the bowl from the intake, then the siphon break goes between the pump and the bowl. If you have a toilet, like the Groco K and many electrics, that don't have a hose between the pump and bowl, you need to use an electric siphon break like Ericoh88 describes.
You can put one in the discharge line because that is not a suction line.
There is a drawing on the Jabsco website that shows this. Unfortunately I can't get on their site right now or I'd post it for you.
Michael, cover the vent with a piece of tape, I bet the whistle goes away and you can draw raw water again. Then move the vent to the correct location.