It sounds to me as if that might be the vacuum reservoir line. The vacuum reservoir is located behind the left wheel. If that is what this one is, be careful about whether or not it holds a vacuum. It is a BIG reservoir and I took it out completely (not a fun thing to do with a Saturday afternoon), looking for a hole or some other reason for it not to hold vacuum. Then I tried it again after I had taken it out and I had to continue pumping the vacuum tester for a good while, then finally showed some vacuum holding.
My climate control vacuum leaks were related to the vacuum switches, of which there are 6 in various places behind the dash. Check the service CD for how to check. It's pretty thorough.