Measurements have been made of the attenuation of second sound in a rectangular rotating resonant cavity. The resonator can be tilted about one of its symmetry axes so that second sound is propagated at various angles ϑ to the rotational axis. The attenuation as a function of ϑ is in good agreement with the predictions of Hall and Vinen's equations of motion for helium II. However, the absolute values of the mutual-friction parameter B are about 20% below previously reported values. In the course of the experiments it was found that second-sound attenuation in a resonator made of porous material depends strongly upon whether the resonator is surrounded with a shield rotating with the resonator or is sloshing in the helium bath. In the latter case it is shown that isotropic superfluid turbulence is produced inside the resonator.