Tuned mass damper would be my guess:
http://en.wikipedia.org/wiki/Tuned_mass_damper
"Tuned mass dampers are widely used in production cars, typically on the crankshaft pulley to control torsional vibration and bending modes of the crankshaft, on the driveline for gearwhine, and other noises. They are also used on the exhaust, on the body and on the suspension, as in the 2CV example above. Almost all cars will have one mass damper, some may have 10 or more."
In my college mechanical lab we were given a single-cylinder compressor that practically jumped off the ground as it ran. We had to design and install a tuned mass damper. The compressor barely vibrated once properly dampened.