Clunk when rolling back

10 million dollar question!! Wondering if anyone has any idea this noise is? I pulled up to my drive today and had to roll back to let someone past (on a slight hill) it rolled back at about 5mph and clunked twice, like clunk.......clunk, about 2-3 seconds apart almost rotational. Any ideas guys? Never really noticed it before and it didn't seem to do it driving forward?
 
Top mounts normally do that as you creep forward or back but normally one unless you pressed the brake to check you need to keeps the weight of the car on the wheels so don't jack it up this is what your looking for.
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If not that then the front pads might had a bit of slap in the caliper although not twice failing that the ball joint or engine mounts is the best guess, drive shafts maybe again was the noise up front high up or down low.
 
I took the car to the garage today for new tyres and the mechanic couldn't see anything wrong, he thinks it might be the hand brake sticking. We shall see?