Does the sound change when the engine is ice cold versus warmed up?
Piston slap is pretty distinct.
Cheap timing chain slap?
How many miles on the engine and do you know anything about the internals?
A wrist pin coming out would be bad but you may find something premature in the oil or the oil filter… If something on top of the Pistons has come loose you might check all your spark plugs.
Torque converter or flexplate bolts transmit sound all over the darn place when they start to come out and tap on things. It is easier to hear them though when you're underneath the car.
Hopefully it is not catastrophic! Let us know what you find!
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