I’m a CFII and have experienced the “phantom flutter” in both C150’s & C172’s with 40° flaps in a full forward slip. Slightly unnerving the first time (feels similar to, but much less violent than driving over the asphalt vibration strips on the sides of highways), but easily rectified by slightly easing out of the slip to more like an 80% full slip... I do agree that the best approach to get proficient deadsticking is by slipping with 20°- 30° flaps and then be dropping final flaps as you’re easing out of the slip...