Take a piece of hose long enough to reach from the end of your hand to your ear. That will be long enough to stretch straight from the valve cover(s) to your ear. Start the car and let it idle. While holding one end to your ear, keeping the hose as straight as possible, move along the valve covers. This will help you better locate the source of the noise.
I suspect from what you wrote that one or more rocker arms may be worn through. I've had this happen with a similar sound. The valve keeper was mushroomed so the valve did not fall into the cylinder (luckily)!
After removing the valve cover then spark plug, I was able to use an air hose to pressurize the cylinder, remove the bad rocker arm, use a valve spring compressor to R & R the valve keeper and spring cap, replace the rocker arm, spark plug and re-adjust the rocker arm.
This method wasn't too hard. I hope it's a simple fix.