Eric, I have to check the c.converter like you suggested. I forgot to mention that when I start the cold engine on my 2000 Lincoln Towncar it hesitates like it is trying to briefly stall every 5 seconds while the high pitched whining lasts, so I give the engine more gas. When I drive it the oil light comes on the screen until it warms up a lot.
Your theory about BOTH cars having the same problems with the same left c.converter whining sounds strange, because the Lincoln only has 85,000 miles on it and the Mercury has 187,000 miles on it...It is possible though, since both cars have the same window switch problems, both have the same 4.6 liter engines, and both are Fords. Is it possible BOTH Fords have the same small piece of broken honeycombs, too, if it's not the seams or heat shields. I will listen to those areas tomorrow, so you might want to think all this over until I can confirm your possible thoughts on the matter.
Also, one friend mechanic, Kelvin, suggested it might be a narrowing of the opening into the c.converter (carbon build up in the exhaust I suppose) squeezing the air flow and causing the whining. My brother saw a lot of white smoke coming out of the 1995 Mercury tailpipe yesterday when I accelerated up to 35 MPGs and dissipated as I got above that, or the white smoke may have dissipated by increased outside air around the tailpipe. Maybe those 2 symptoms coincide with Kelvin's idea...
A 2nd mechanic, Tim, said he had a similar problem on a different car of his, and it turned out to be a vacuum break due to a hole in the gasket leading into his c.converter. I have 2 c.converters on BOTH my cars. What do you think of that idea?