Have you replaced the rubber grommet in your valve cover? They are notorious for drying out and shrinking even if they look fine visually. A new PCV valve will seal for awhile then once she gets a little oil around her which does not take long will start moving around in worn out grommet. Went thru whistle/whine pcv issues on my 65 cuda & after several pcv valve replacements finally replaced rubber grommet on my valve cover with new softer rubber one that sealed nicely and no more issues.
