I've never heard that about the L-82 of any year, but I s'pose it's possible. The C4 model L-98s are BAD slappers when cold...just a fact. Doesn't hurt anything on them, tho.
Does yours only do it when cold, or does it have this noise all the time?
IF all the time, does the noise change when it warms up, or stay the same?
Does it sound like only one piston making the noise?
Piston "slap" is what happens if there is a little too much clearance between the piston and the cylinder bore. When the piston reaches TDC(top dead center) OR BTC(bottom dead center), it will rock in the bore, and create a noise as the piston barrel hits the bore wall...kinda like a diesel engine running, but not as loud, or constant. Most of the time, this doesn't hurt anything. If it is really bad, it could eventually break the piston.
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