It defenitly sounds like a leaking vacum line, seal or connection making the sucking sound. Hence when you introduce another source for the air to get in the sucking sound goes away since the air intake will take the easiest path. I don't think the occasional knocking sound is related. If you have a steathascope or chunk of garden hose you can try and pinpoint the source of the sucking sound.
keep in mind the sucking sound may not be the source of the problem but rather the easiest path for it to draw air in. Could be a plugged air filter or breather somewhere forcing the air to be drawn in from somewhere else.
Sorry I couldn't be more help.
keep in mind the sucking sound may not be the source of the problem but rather the easiest path for it to draw air in. Could be a plugged air filter or breather somewhere forcing the air to be drawn in from somewhere else.
Sorry I couldn't be more help.