Using an electron beam probe, the existence of a standing electrostatic wave in the sheath surrounding a planar probe immersed in a plasma when the probe is driven at frequencies near ωp has been demonstrated. The driven probe exhibits two admittance resonances. The larger of these (near 12ωp) is the well documented “resonance probe” resonance. The smaller second resonance at ω≈ωp is associated with the standing wave and its interpretation as a Tonks-Dattner resonance is discussed. The effect of such a standing wave on the usual interpretation of plasma resonance probe data is examined.