The ornamental design for a built-in ventilating fan, as shown and described.
1. Built-in ventilating fan
1.1 : Perspective
1.2 : Front
1.3 : Left
1.4 : Top
1.5 : Bottom
1.6 : Perspective view 2
1.7 : Front view 2
1.8 : Left view 2
1.9 : Top view 2
1.10 : Bottom view 2
The broken lines depict portions of the built-in ventilating fan that form no part of the claimed design; the dash-dot broken lines define the boundary of the claimed design and form no part thereof; reproduction 1.1 is a front perspective view of the built-in ventilating fan; reproduction 1.2 is a front view thereof where the back view is a mirror image; reproduction 1.3 is a left view thereof where the right view is a mirror image; reproduction 1.4 is a top view thereof; reproduction 1.5 is a bottom view thereof; reproduction 1.6 is a front perspective view thereof in a condition that is in use; and reproductions 1.7, 1.8, 1.9 and 1.10 are front, left, top and bottom views of reproduction 1.6.