When I shift gears it is difficult to do so sometimes and there is a loud noise

What is your question?

When I shift gears it is difficult to do so sometimes and there is a loud noise when I shift from park to reverse and I don't know why. It only happens maybe 1 out of 50 times or so. But when it does it is strange it is hard to shift into reverse when it does that. It seems like it is stuck in Park. And I cannot get it out unless I put a lot of pressure on the gear shift and then that is when I hear the loud clunking sound when it shifts from park to reverse. Does anyone have any idea why.