[bringselpup]

This summer in brutal heat while driving if I accelerated, went up hills etc. I'd get the vent change to defrost thing. I eventually found a crack in the vac line right near where it went into the firewall to get connected to the hvac stuff under the dash.

I cut it clean both ends, had barely enough coming through the firewall, and spliced it with a piece of RC airplane fuel line hose.

And I found it by hooking my manual vac pump for bleeding brakes up to the lines and heard the noise. Never would have seen it cause it was covered with crud.