98 lesabre noise from fuel tank

hi i had to change gas tank and sending unit with fuel pump old one (rusted out) i got them from the scrap yard they looked new, anyways i installed on car and every thing works fine except i get this thud noise from fuel tank every once in a while like metal is contracting and causing a hollow thud sound, has anyone had this problem or could it be a problem with fuel tankk? does anybody have any ideas.

hi i had to change gas tank and sending unit with fuel pump old one (rusted out) i got them from the scrap yard they looked new, anyways i installed on car and every thing works fine except i get this thud noise from fuel tank every once in a while like metal is contracting and causing a hollow thud sound, has anyone had this problem or could it be a problem with fuel tankk? does anybody have any ideas.

When does this thud happen? Is it a one-time occurance? Or does it happen multiple times during a drive?

When does this thud happen? Is it a one-time occurance? Or does it happen multiple times during a drive?
hi the noise happens multiple times and it seems to happen when going up or down a hill ( or when fuel is shifting ) but not absolutely sure at this point i thought it was something in trunnk and i tried putting on brakes and releasing them to see if i would get this thud noise but it seems that at this point it is not consistint.there is a bafle inside fuel tank i wonder if that has something to do with it, on old tank baffle is made out of fibre glass on this tank it is made out of metal is this normal on after market tanks.