I also used the nail trick to hollow out a space. Slid in some popsicle sticks on a string and filled with thickened West Systems. I then drilled and set in stainless steel Helicoils. Just over two years and they are holding strong.
I had the same problem. It is most likely coming from the valve cover. Try tightening the large screws holding the valve cover down. Mine work loose every so often.
I just had a similar issue. Check your valve cover bolts. Mine had worked their way loose. Oil dripped over the edge and pooled near the top of the motor mount.