Toyota Repair: transmission whinning noise

Question
Hi Ted,
1992 Corolla 109,000 miles. I recently was having a high pitched whine noise coming from the transmission. It only occured when the transmisssion was warmed up. ( trans fluid changed regularly and trans fluid looks good. Although it was over filled for a couple of monthe.)  I thought the transmission was going bad. I looked back at my maintenance records and noticed the noise started right after I changed the trans gasket cause it was leaking - and also the trans screen/filter - which I bought at Auto ZOne.  So I again changed the screen/filter with a better one and put in some synth trans fluid.  To my surprise it stopped making the whinning noise about 90% but still makes the noise when the weather gets warm and when the trans warms up. ( the better trans screen has about 2 times as much area where the screen is open to allow fluid to go to - i guess to the the fromt pump.) I am trying to keep the car as long as I can. Should I add a trans cooler, should I flush out the trans fluid and put in all synth trans fluid?  Do you think these things would help or am I just wasting money - figuring the trans will go bad someday?  
Bob

Answer
Hello, the whine noise usually originates from the differential gears and some is normal considering the age of the car, a trans cooler keeps the temperature of the fluid down and so the fluid will last longer, heat is the #1 enemy of automatic transmissions, flush all fluid and replace it with synthetic completely as the regular dexron and synthetic really are not compatible, you might also consider adding some kind of transmission treatment, I understand that Lucas makes a very good fluid additive.