2006 yz450 knocking sound

I went to check out a 2006 yz450 today. The bike is in good condition, cranks after 2-3 kicks but when idling it makes a knocking sound like hitting a hammer on steel. The owner says all his friends 450 make the same sound.I don't notice the sound as much when you rev the engine, but could be exhaust noise drowning out sound. The bike seems to have good power. The bike has 71 hours on hour meter. He doesn't know if previous owner had adjusted valves but he hasn't. Is this sound normal or is something inside ready to break? Thanks for any  help on what this could be. :ride:

Compare it to two others.  The YZ450 is notoriously noisy.  It's a purpose-built race engine with no effort wasted on niceties like quiet operation.

also may have had s.s. valves put in sometime or another ...when I put them in my 04 the noise change was crazy sounds exactly like what your saying like 3 tiny hammers everytime my intakes close??? very concerning to listen to but ive had no issues thus far

Thanks for the suggestions. I listened to a few other bikes and the sound was similar. Got a good deal on the bike so I bought it.

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