I have a 2003 Buick Century in automatic with overdrive. During the past week I experienced a hum and slight vibration at speeds over 20 mph. (The faster I drive the higher the hum.) I checked the transmission fluid. It was brown and pink but did not smell burned. I have not changed the trans fluid in 2 years. I sent it to my transmission guy for an oil change. He said that the 1st gear clutch was having some problem and there was no 4th gear. He suggested another transmission…what do you think?