"tampon" being the cover of a manhole, etc., not the manhole itself.

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Note added at 43 mins (2007-09-07 08:47:41 GMT)
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If for cleaning, you might want to research in the direction of "cleaning eye" or "rodding eye". That is certainly said of drain pipes, and is what the PVC things in your pictures would be called. Don't know that cleaning is an aspect of HVAC systems, and certainly not rodding, but it is possible that the insides of air ducts have to be wiped down periodically to remove layers of dust. The sort of thing that is recommended, I imagine, but never actually done! Don't believe those images you see in films when people escape through air-conditioning ducts that are bright and sparkling on the inside!