What can cause the engine to make a noise like the engine is full of rocks? on 1991 Buick Regal

Vehicle stalls out and is hard to keep cranked. Sometimes I have to keep the accelerator slightly engaged to keep vehicle from stalling. I have replaced the throttle positioning sensor. Also the engine has a rattle. Sounds as if the engine is full of pebbles when it is running. What is the possible cause?

Is the low oil pressure light on? Oil at correct level? Or broken-up catalytic converter rattling?
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Sounds like you have internal engine damage. Possible rods, etc. Either way you will have to open up motor to see exactly and repair it. This is better served at your trustworthy auto repair shop. This wil cause loss of compression which will cause the issues you are having.
sounds like you spun a rod bearing,I would pull the oilpan to check for play in the rods