Still think it's valve tick, typical with a motor of this mileage. Mine sounds about the same if not a little louder. For warmup, just keep RPM under 2000 till blue warmup light disappears. If theres enough piston to bore clearance for it to cause audible piston knock it should cause mechanical failure before too long, if it was assembled at the factory out of spec like that, it wouldn't go this far without total failure. I would get a mechanics stethoscope if possible or just use a long screwdriver, listen to the motor while idling and put the tip of the screwdriver on the valve cover and put your ear up to the handle, listen to it and move the screwdriver to different parts of the cover to see if it's just a couple valves/lifters that are making the noise.