Clear spots on negative visable only when viewed at an angle, usually do not print.

I have recently noticed a large number of dust-like clear spots on my black & white negatives, but they are only visible with the negative held at an angle to a light source. The spots are not visible when light shines directly through the negatives, and they usually do not print. (I say "usually" because I have noticed on rare occasions black spots on prints that I cannot explain.) I looked at some older negatives (different film type and different developer, but all Ilford) and saw the same thing. I only now have accidently discovered this while viewing a negative with a loop. Could this be caused during manufacturing? Any ideas?