Usually, the fluttering has been reported on the OPN and OPN S possibly due to the Feedback Shield being enabled. Feedback management usually employs 3 strategies, frequency shifting (by 10 Hz for the OPN), phase cancellation, and gain control. Usually the frequency shifting is the culprit for the fluttering you hear, especially on single tone type sounds. Unfortunately you can’t selectively turn on/off each of these 3 strategies. They’re all bundled together and Genie 2 decides how the feedback control is going to be deployed if you run through the Feedback Analyzer and deploy/enable the Feedback Shield.
At least, if the Feedback Shield feature is enabled on your OPN S, you can try disabling it to see if that is really the cause or not. If it’s confirmed to be the cause, then maybe you can set another program similar to your default program, except that this new program will have Feedback Shield disabled. It’s not ideal because you’ll have to give up one or the other, but at least you have set up to choose either or by changing programs.
The OPN S, beside having the traditional Feedback Shield feature that’s been around even before the original OPN, also has a new feedback prevention technology called the OpenSound Optimizer. It analyzes the incoming sound and determines the potential for feedback, and does some trick to avoid feedback in the first place. As opposed to the Feedback Shield feature, which is a “reactive” technology, the OpenSound Optimizer is a “pro-active” technology which heads off the feedback from happening in the first place, instead of waiting for the feedback to happen then reacts to it. However, a number of OPN S users have reported some fluttering issue with this technology as well. Oticon is aware of it and is supposed to be working on a fix. However, I haven’t heard of a fix yet and it’s been at least a year since the issue was reported.
So your fluttering issue may be due to either the traditional Feedback Shield, or the new OpenSound Optimizer. At least you can turn off the Feedback Shield (if it’s turned on) to see if it helps. If it’s not turned on, or if turning it off doesn’t help, and it’s likely an issue due to the OpenSound Optimizer and all you can do is wait for the fix. I don’t think you have the option to turn off the OpenSound Optimizer, but I don’t own the OPN S (I wear the OPN 1), so I may be wrong.