What can cause this loud embarassing sound? Will headers with several exhaust leaks and no cat cause it? Is is a rebuild engine thing? Is it a crappy gas thing? Could it be a header thing? Or is it all of the above?

Ever since my engine rebuild, my car gives a horribly loud backfire after WOT runs. It seems to quiet down and even not occur after a warm up period, but my drives mainly consist of 5 miles. I also added headers during the rebuild process and eliminated the cat as well. I haven't had the money to get a nice cat+back system, but would that eliminate the backfire?

The exhaust leak is coming from where the cat used to be, I just got some cheap $5 flexpipe and clamped it on to the catback and header y-pipe. It's embarassing, I feel like Uncle Buck.

 

a bad enough exhaust leak will most diffinitely cause a back fire at WOT. If you do not want to be like "uncle buck" than do something uncle buck wouldn't have done.

Get it fixed.