Wild Mushroom Noodle Soup

Submerge porcini mushrooms in 3 quarts cold water in a large bowl. Let stand at room temperature at least 2 hours or chill overnight.
Pour mushrooms and liquid into a fine-mesh sieve set over a wide heavy pot, gently pressing mushrooms to extract as much liquid as possible. Reserve mushrooms for another use.
Add garlic, thyme, bay leaves, and peppercorns to pot and bring to a simmer over medium-high heat. Reduce heat to medium-low and gently simmer, stirring occasionally, until broth is reduced to 6 cups, about 1 hour. Strain broth through fine-mesh sieve into a large bowl; discard solids. Return broth to pot.
Cook noodles in a large pot of boiling salted water, stirring occasionally, until just al dente, 7–8 minutes. Drain; rinse under cold water.
Meanwhile, bring Mushroom Broth to a simmer over medium-high heat. Add wild mushrooms, carrots, celery stalks, scallion whites, 1 tsp. salt, and 1/8 tsp. pepper; simmer until tender, about 5 minutes.
Add noodles and scallion greens to soup; season with salt and pepper. Transfer to a large serving bowl and top with celery leaves.
Soup (without noodles) can be made 3 days ahead. Let cool, transfer to an airtight container, and chill.