Engine noise while gaining rpm

First off, please excuse my ignorance and if this has been covered before, I couldn't find it anywhere. I've got a 2010 yz450 and I've noticed on the past two rides that I'll occasionally get a slapping sound in my engine when I gain rpms after lugging. I was wondering if this is my timing chain or piston slap. I initially thought my drive chain was slapping, but after listening closer, its in the engine. Thanks in advance for any help.

Edited by cover1033

Rear chain slap.  The timing chain cannot sense engine load, so it doesn't respond like that. 

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