My F makes a strange whirring/chirping noise at 1500-1700 rpm. This only happens once the engine has warmed up, after 20 minutes or so. The noise seems to come from somewhere behind my right ear. I took it to the garage, but they "found nothing". I suspect they didn't run the engine for long enough to warm up. Has anyone experienced this? I don't think it's a big issue, but it's irritating. Thanks!