A new type of low-frequency mechanical oscillator was developed to study the vortex lattice melting transition in untwinned single crystals. The dissipation associated with the oscillatory motion of the sample was measured as a function of temperature down to 77 K in magnetic fields from 2 up to 10 T. A sharp changeover in the dissipation was identified as a transition signal. The data were analysed with a linear ohmic response model. The transition signal at low fields was interpreted as a peak effect of the critical current. The peak effect vanished at high magnetic fields.