Buick/Olds: 98 LeSabre transmission, buick lesabre, trans fluid

Question
'98 Buick LeSabre 3.8L with 120K miles.  
No previous problems or repairs.
After driving at night in rain for about 200 miles the car developed a noticable RPM dependent whine.  The transmission started shifting real fast, meaning it would realy snap your neck when it shifted.  It was shifting at the right time and there was no delay and no slipping.  
Checked it this morning.  The problem has completly disappeared.  There are no leaks and fluid level is right.  Fluid is clear with no bad smell.  We have not added any fluid since the car was new.

Answer
What you are describing sounds to me like a problem with the pressure control solenoid in the trans. This is a pretty common problem. The pressure control solenoid "sticks" causing the trans to run with full line pressure. Thats what causes the harsh shifts and whining noise. Usually it sticks due to dirt in the trans fluid.
You should have the PCS replaced, trans fluid flushed, and a new filter put in. This should take care of the problem.
Also it will start having the problem more often the longer you go without repairing it and it could cause other major damage to the trans if it keeps shifting at full line pressure.