We describe a new resonance of superfluid4He when confined in a superleak. In our experiment the superleak is sealed except at a filling hole which communicates with bulk liquid. The resonance is driven and detected thermally. The restoring force is provided by the compressibility of the helium under conditions which are very nearly adiabatic. We derive expressions for the resonance frequency, the lineshape and phaseshift and compare these with measurements on a 0.2113μm superleak.