1990 g20 sportvan 5.7L four speed automatic transmission,3/4 ton. When I'm driving and transmission is in fourth gear when I start to go up a slight incline my transmission starts to make a noise,when it down shifts the noise goes away I need to mention that this only happens when I'm going slow like 30 miles an hour. On the road no problem I'm thinking that its like too slow for fourth gear? Thanks Bill