Greetings Marie,
Assuming these hail-like nodules are not mature acorns, they could be either immature, first-year red oak family acorns or tiny galls caused by parasitic wasps, mites, or other insects.
Either way, I really don't see this as an immediate tree health concern. Without a visual observation, I really can't diagnose this problem with any degree of certainty. Therefore, if this is a major concern of yours, then I would strongly urge you to contact your local university extension office.
Thanks for your question!