Hi, I can only offer you 'generic' pointers as I have the K12.
Even on the K12 with its notorious box O rocks idle rattle (that's caused by the intermediate crank gear slopping about), rattles under power should not be there.
That is because the intermediate gear is under load and the cam chain should be running nice and tight. Of course, if the cam chain tensioner is not working or the guides are worn then you could get rattles under load.
I don't want to worry you, but I had this problem once on a car engine and it was caused by low oil pressure. Although you have an oil pressure warning light, it only comes on at around 7-10psi. You would need to temporarily find a way to attach a gauge and check you are getting around the 80 psi oil pressure at 2K rpm.
Another issue might be pre-ignition or 'pinking' under load which happens when a motor is timed too advanced for the fuel it is using. There are many new lower octane fuels around now and with an older bike may need to consider retarding its timing a degree or so if it is still at the oem setting. If you find the idle is not stable and fluctuating, this could be the cause.
Another cause of rattle under load is top end piston bearings and/or lubrication issues, a similar thing is an oil pump at low pressure gets less oil to the valve and cam gear.
Don't get paranoid, but do check the oil presure first.
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