2002 XR650R- loud tapping/slapping noise in motor.

i normally road my 650r 4 to 5 times a week about 2 hours a day for 6 months. i brought my bike home and it sat for 4 or 5 days until i started it again. when i started it it had a loose metal clanking noise in or around the engine, i turned it off to do a visual then i fired it up again. as soon as i started it the second time it was tapping/slapping noise very loud. i stopped it roughly within a minute. my bike has compression and is able to start after a few try's.
knowing that it hasn't seized is my only comfort at this point.
i checked the oil after this and realized i was running 1/2 qt. low. i drained the oil and put 1 1/2 qt's. i checked the oil for any metal or debri and couldn't find any.
anybody have any suggestions?

What did it sound like after adding/changing the oil?

Undo the decomp-lver's cable first, and push the lever on the head there where it has to be - any change?

If not then your best bet is to take the rocker-cover off - AFTER you measure the valve-play first!
Check the camlobes & followers for wear, plus check the play on the various shafts (by feeling).
If all looks fine then I'd take the jug off, and see what gos for what. Feel the conrod's plays (up&down, sideways & sideways-rotational (like a clock-hand)), and of course check for any visual damage on the piston & liner.
If this is not it then remove both side covers, and feel the crank's bearings themselves.

The above should tell you if something's wrong, with only a few gasket-cost & work.
If not then dig deeper.
Let us know what you find?