First things first.

Is it a tap ... like a rhythmic beat? Or an actual banging? If the
former, thats normal after they build up gunk. The Mitz stock motor
oil ports are too small on the lifters. Not to worry -- this is not a
problem ... At the next oil change pour in some transmission flush,
let idle for 10 min then drain oil and refill w 3 quarts  5W30 + 1 qt
Lucas oil stabilizer.

If its a loud (like really really loud) banging sound it could be
loose bearing connections at the crank causing a slapping noise. Again
repeat as above but add 2 qts 5W30 + 2 qts Lucas. That should quiet it
down some and forestall further damage til it can be fixed or sold.

But don't go buying a new motor until you figure out that it's a
busted engine for sure.

BTW, these motors can be rebuilt while still in the engine bay, so it
could be cheaper to do it that way.

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"Take a little 5FU, leucovorin and irenotecan for thy stomach's sake." -- 1 Timothy 5:23 (adapted)