98 Yz400f - Noise

I have a 98 yz400f- possibly looking to rebuild me top end. Last summer it had a weird knocking noise the sounded like it came from the top end. Almost like the piston was knocking on the cylinder head. When the bike idled it would get loader then decrease in sound. I did go away after the bike is warmed up but not totally. Its something i would like to take care of before it may cost me more money. Any pointers on rebuilding top end would be appreciated. Thanks

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