Transmisión problems from 1st to 2nd 2001 silverado 1500

[__sergii]

New guy here, not sure if this is the right sub forum to post in. Please move if it doesn't belong here.

So recently when I'm moderately pressing the gas pedal. I feel like a small thump when the truck changes from 1st to 2nd gear. Just a small thump and nothing too major but would still like to get it fixed.

This doesn't happen when I'm pressing lightly on the hasn't pedal

The truck was handed down to me with 136xxx miles and recently had to change the fuel pump.

 

Any help/advice would be truly appreciated. Thanks

[silveradosid]

lube the yoke spline going into the transmission