Sinusoidally squeezing a capacitor
A practical plate capacitor  has a part of its field bulged out of the gap and that part becomes more significant as the distance of the plates grows. The bulged part doesn't stay constant if the distance of the plates oscillate. That generates oscillating magnetic field as J.C. Maxwell wrote in his famous compilation theory of the electricity. It can also cause significant radiation if the gap is long enough and the gap oscillation frequency is high enough. Otherwise a charged but unconnected capacitor (even no wires) acts like a tensioned spring which dissipates nothing when one pulls the plates further apart.
If there's wires the system is complex. The wires are a load and there's AC current between the plates and wires. That causes losses if there's resistance and the needed mechanical oscillation frequency for substantial radiation becomes lower although not necessarily practically realizable.