how do i know if the abs pump is the problem?

Its cause when the truck is going at about 20mph the abs light turns on and then the truck makes a sound.

Don't think it's the pump ...most likely a wheel speed sensor but....see answer #2.. That's the only way to find out!!

And what could be the reason for the truck making a noise?

When an ABS wheel speed sensor... or it's circuit..., fails, the ABS module thinks one of the wheels is 'sliding' or locked....so it activates the Antilock Brake System to release that wheel.. .. That's what makes the noise.... So, now with that said.... Have the ABS scanned to find out what is causing this problem as already suggested!... No way to guess at it.... that helps!

And do i need to replace the module for the noise to stop or what can i do or is the noise normal?

Park the truck and ride the pony!!!!

OR , they could s l o w d o w n . . . . and READ your explanation , which was pretty clear. . . . . What's a pony???

Screen name was ''ponylover12".....????? .Must have changed it........ I would have too, ... now that i think about it!!