What is the cause of the ringing sound my client [name withheld] hears in his head and what more may be needed to stop it?

This was caused by spirit meddler attachments and not an extraterrestrial implant. If you think about this, you will see this is obvious because what good is a secret implant if it announces its presence in such a dramatic fashion? So whether it changes will be a function of effectiveness of the healing. This has been initiated but will not necessarily be sufficient at first to stop this symptom. Time will tell. Revisiting him with the application of your Protocol will, in fact, do the job, but may take more effort than you are willing to expend for that minor annoyance. This is at your discretion, of course.