Could be piston slap if the skirt clearance is too wide.
Could be spark knock.
Could be the PTO.
Could be the driveline.
Could be ............
It takes a keen ear and some experience to know difference between the sounds. I'll say what I normally do in this situation, take a video and post it to a hosting site (Youtube) and we'll have a listen. Maybe it's normal, maybe it's not.
Make sure to retorque your head bolts if you didn't. Supposed to retorque them after a good heat cycle of the motor.
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