Couple of times boom got locked out while operating and pressed park then unpark switch and it unlocked the boom and continued to work, has hand controls. Now boom is completely locked and will not unlock. The machine starts and moves but the boom does not unlock. When I press the unlock switch I feel a thump at my feet and the boom light flashes then stays lit and the boom controls are locked. Not the typical seat belt or seat interlocks as they release and am able to drive it. At a loss for what to check. Suggestions?