John:

Thanks for the very informative response. The engine was blowing a lot of oil through the oil filler cap. Even doubling the rubber gasket under the cap did not stem all of the blow by. I had imagined that the excess pressure was related to worn rings. Yesterday the PCV valve was removed, doused with a quarter can of carb cleaner and shaken (not stirred) violently for a minute or so. Afterwards there was a whole lot more rattling going on in the valve, even with gentle shaking. Today the valve cover was nice and clean after some highway driving with no evidence of oil seeping past the filler cap seal.

This morning the engine was quiet again even though it was 20 degrees F. The gremlin must be waiting for a less opportune moment to reappear. I really do suspect that it is an exhaust noise, but my thinking is surely clouded by expectations.

I'll post again when more definitive info is available.

Thanks again for all the help,

Joe M in WV