What causes the BANG when an airliner lifts off?

I've flown a lot of planes, of different sizes, and I always lift off gently.
When I fly as a passenger on an airliner though, there is a "thud" or even a
"bang" as the main wheels leave the runway. Sometimes this bang is so loud is
frightens passengers, and once it was strong enough (on a 777 departing for
Hong Kong) that I was sure we'd at least blown a tire, if not actually struck
something with the mains. But in the end, it's always OK - just normal. Who
knows what this is? Is it the pusher actuators on the main gear? The
telescoping elements of the main gear bottoming out? Just curious. . .

G Faris