So at about 1400-1700 i get this knock, sounds like its coming from under the car like the transmission perhaps, and this is only in first gear. the slower i accelerate from stop the lighter the bang is, almost nonexistant, but you feel a little bump. When I give it about half throttle it bangs much harder. I did just replace my rear section with the cattman, but I don't think its the cause.

any ideas what I should look into?

 

When you say knock, what do you mean exactly? Like does it sound as though it's metal knocking around or is your shifting affected? Because what it sounds to be is that your cat is loose and is knocking around, if that's the case then you need to have the bracket under your b-pipe checked and or re-welded back up.

 

well its in the rpm range i said, and shifting isnt affected

it basically sounds like metal on metal hit. and it just happens once, in first gear, around 1300-1700 rpm