It can often be difficult to analyze problems based on verbal desciptions over the internet. It does sound like you may have an internal transmission problem. You could have it removed and overhauled or replaced, or you could wait and see what happens. The issue is: Will driving it until there is a hard failure cause consequential damage and cost more in the long run. That's the risk you have to evaluate versus the cost of overhauling or replacing the transmission now.