[derekgas]

I heard something like this a few years ago panther, it is somebody 'piling' foundations in nearby property, you can time it if they are any good, it is a hole drilled into the ground, then they drop a huge weight down the hole until it is well compacted, then pour concrete in, the space in time between noises is while they hoist the weights back up to drop them again, the vibrations can travel hundreds of yards.