Mild backfiring...

I swapped out my headers today from the junk yard (if you want to see pix i can post them) place seemed to rip me off at least it seemed that way cuz they charged me $32, but maybe im gripping too much. Anways, after I finnished changing it out I noticed a weird sound from my exhuast (was probably there before hand, just didn't notice it) and my friend did the dollar test and you could see the dollar every so often get sucked in a lil bit. What do you guys think it is? Maybe a timing problem, I know the previous owner messed with the ignition timing and gave the valves a much larger lift. I mean overall the car runs fairly well, just i rather fix this problem if it is something simple. I'll probably take it to my muffler shop to see what they think it is. Thanks in advance for any info :iceslolan