This is strictly a guess, but it probably has something to do with the picture here.   This is a photograph of a typical “pit can”; or in industry terms, the oil-collection means.   Most hydraulic elevators are equipped with a 5-gallon bucket (as shown) that is designed to collect the overflow when the jack seal leaks.   The hose in this photo runs from the jack head to the pit can.  When the jack seal (or “packing”) becomes worn, the oil that pushes the piston up through the cylinder begins to run past the seal and onto the head of the cylinder.  When this happens, the oil is then collected through the hose and into the pit can.

Most hydraulic tanks (not pictured) are filled to the minimum volume + 5 gallons.  When 5 gallons of hydraulic oil has escaped the system and subsequently travels into the pit can, the system will collect air and will begin to cavitate.   The “grinding” sound you hear is always at the top floor, and most of the time this indicates that the pit can is full and needs to be emptied back into the reservoir (or tank).

Some other causes are noted here.

If you’re riding in an elevator and you hear an awful sound as the elevator approaches the top floor, you now know what that could be.   Usually the jack seal will be scheduled for replacement if the pit can fills up once a month.   If you hear a grinding sound when riding your elevator, notify the building manager and instruct him/her to contact the elevator service company to check it out.   If this sound reoccurs more than once a month, take the initiative to notify your service company’s manager directly and inform him/her of the situation.