I was doing well with my 6S Plus but either it has developed a slight rattle on the earpiece or I just didn't notice it before now. It is slight, but I have kind of tuned into it. It is mainly female voices that are quite resonant that seem to trigger it. I found someone else with a similar issue that used the pre-recorded press1,press2, Tesco Clubcard Freephone number as a test for it (0800 591 688) and sure enough that does stimulate the rattle on my phone. Most conversations it happens barely so I wouldn't notice it, but there is the odd voice that makes it really irritating. It is not a blown speaker and it doesn't sound like a loose speaker. It sounds like a resonance issue. For that Tesco Clubcard one, I would have to turn the volume down to about 1/3 to eliminate it. And having it 100% or 60% doesn't make much difference...in fact if anything at 100% its better because the rattle doesn't increase but the volume of the person talking does, making it less noticeable.
I found that if the call is an "HD" voice call it doesn't happen...and if I switch "hearing aid" on in accessibility settings which I think filters out some of the resonant peaks, it gets rid of it too.