French term (edited): qu’il insérera dans la gouttière du projecteur

I think you'll need to re-word slightly, as the writer appears to have got the technology slightly wrong.*

When I was a projectionist, we used to have to do this all the time, and for a similar reason: we inserted a small piece (of fag packet, usually!) to interrupt the light beam as it passed through the film and slit, where it passes (usually) round the 'capstan' of the sound flywheel (depending on the exact design of each specific projector).

So in this particular instance, it isn't that actual picture gate (inserting a piece of cardboard there would not help the sound at all!)

MM is exactly right in his explanation in the discussion box above.

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Note added at 8 hrs (2010-10-25 22:09:39 GMT)
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By the way, they're not really 'captions' so much as 'titles' (since here we're talking not about the caption card that is filmed, but rather, the resulting 'title footage' which would have to be intercut with the main films.

It sounds a bit as if this is 16 mm, since if you use double perf. film stock, this can produce a horrendous noise when run through a projector with optical sound, as the second row of perfs. is in the space normally occupied by the soundtrack.

35 mm, on the other hand, doesn't suffer from quite the same problem, as it is always double perf., and the soundtrack area is reserved. But unless specifically exposed to be black (= opaque), it will be printed as clear, which can lead to this sort of problem (particularly, the sound of splices going through)