Airbells pinholes & white spots

I've had recurring problems with negatives and need some help identifying the culprit.

I can't post a picture so will do my best to describe the problem.

The emulsion has small "pinholes/ airbells" at random parts of the negative.They are usually light grey with a black ring around the edge and are approx 1mm diameter.

Efke 50 120 and possible other films.
Paterson Small tank
Rodinal 1+50
Water stop
Iford rapid fixer 1+4 for 5 mins max
2 inversions every 30s for the processes with continuous agitation at the start of each process.
Sometimes I tap the tank on a hard surface.
20 min wash in flowing tap water.

I don't pre-soak, but may try this.
I've cleaned out the film back.

The black rings are the interesting part, so does this suggest a problem at the developing stage?

Have you noticed any particulates floating in your water and/or developer? If the developer has any chemicals precipitated out that sticks to the film it might cause what you are describing. Otherwise it sounds like an emulsion defect.

Airbells and pinholes are generally clear small spots on the negative with no ring and no darkening. I think Gary has hit it. Undissolved material will cause dark rings and dark dots.

PE

So oxidised Rodinal?

Undissolved chemical specs or something in the water?

PE

I've used two different locations for the water supply so I can rule that out. My agitation technique is good and I always use pre-prepared liquid chemicals.

My recently processed Acros looked good, so I'll hunt out the other negatives and see if it was a dodgy brick of Efke.