Found my clunking noise, what is this tube

Greetings,
I had a clunking noise I thought would go away when I replaced the front engine mount on my 2001 Boxster S. The noise remained. At a suggestion on another forum, I replaced the sway bar links, noise remained. When I was bleeding my clutch slave cylinder (what an awful job), I hit and wiggled a small (3/4" diameter) tube that rattled between the heat shield above the muffler and the bottom of the rear trunk. I found my clunk. The tube comes up the driver's side frame rail (unibody) and disappears over the heat shield above the muffler. Can anyone tell me the purpose of this tube and where it might end at the rear of the car. I temporarily solved the clunk by zip tying the tube to the bracket holding the clutch hydraulic line.
Thanks in advance!
Jim

What is the black tube on the left and where does it go?

Grant

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