402 engine problems???

[mleeb]

My BB 402 idles ok but a low thumping noise is present. It increases frequency and volume as rpm goes up. Here's the story of how the noise started... I installed a set of summet headers on the engine a few months ago and the truck sat for a few months till I saved enough to do exhaust( dual 40 series flowmaters with H pipe by the way...) After and before exhaust mods the engine ran good. I drove the truck after finishing the exhaust with no problems until the new headers heated up and melted through some of my spark plug cables...then it ran like crap because the ignition was sorting all thre time. I replaced the cables with a new set with 45degree boots and fiberglass boot covers and replaed all spark plugs. After all that the noise is there...what do you think it is ? what did I break?

P.S. I'm running a MDS 6AL ignition if that helps-