Porosity
What you see as air bubbles are in fact that, air bubbles! They are suspended in the aluminum during the casting process. To the aluminum casting industry this is known as "Porosity".
When I worked in a die casting factory our parts got x-ray'd twice per 12 hour shift just to check for porosity. Porosity can be a very dangerous thing. If it happens on a load bearing member of say a suspension the section of aluminum with the porosity has a much much higher chance of cracking. On a transmission housing casting the section with porosity can even leak. Normally QA standards should prevent parts with porosity from getting on parts. But, as we all know, GM doesn't exactly have the best QA standards or the most reliable cars out there.
What can you do? Well, getting the part x-ray'd would be nice, but who has money or the facilites for that. You could try sanding even more and see if the air bubbles disappear. Just be careful of the intake wall thickness so you don't accidentally sand a hole in it.
etyrrany
PS. We used to do C5 differential housings at this die casting factory. The die was so old that we could only manage a maximum of about 10 parts per hour, and the castings were ugly as hell!
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