Internal or external pump?
A leak large enough to introduce micro bubbles can easily be small enough to be water tight.
Another thing I've seen is cavitation from the input to the pump being restricted somehow. This usually only happens with high pressure pumps though.
If your pump is external go around all the seams with PVC
glue and see if that stops the bubbles.
Also make sure that there's no whirlpool forming inside the sump above the return. This usually only produces sporatic bubbles.