My heat pump just started to make a banging noise coming from the bottom of the unit, also I notice that sometimes there is a dampness around the bottom of the unit on the floor.

How do I get bonded in Texas

You did not actually say inside or outside so you get to answers.

Could be the defrost cycle OR the fan blade hitting ice OR you have liquid slug back to the compressor. I would schedule a service call from a local professional before damage is done.

TURN THE outside unit off till it is repaired

NEXT it could also be the blower wheel or motor mounts on the inside unit. Turn power off remove blower door cover and try to move the motor itself by hand (back and forth). If the motor mount is causing the problem it will move around and cause the banging sound.

It could be the indoor coil being froze up allowing the condensate to drip down and miss the drain pan. Check for a frozen coil also.

Are you hearing this noise coming from the indoor airhandler or the outside condenser unit?