A fluid isolator apparatus (20) for isolating vibration between a vibrating member, such as a helicopter transmission or pylon and another structure, such as a helicopter fuselage. In one aspect, the fluid and elastomer apparatus (20) includes an inner member (24), an outer member (22), and an elastomer element (26) flexibly interconnecting them. First (30) and second (34) operating chambers are interconnected by a passageway (36). Fluid (37) oscillates between the operating chambers (32, 34) through passageway (36). A spring (42) flexibly suspends the piston (40) such that dynamic motion between said inner and outer member (24, 22) displaces piston (40) and spring (42). Preferably the a hollow cavity (28) houses both the piston (40) and a metal coil spring (42). The stiffness of the coil spring (42) may be interchanged to adjust a resonant parameter. Optionally, the piston mass itself or an additional tuning mass (58) mounted on the piston (40) may be used to accomplish tuning. Embodiments describing pistons (40) including rolling diaphragms, elastomer tubeforms, and metal bellows are described.