What's this Annoying, Frustrating, and Impossiible to Find Noise?

Already posted this in '94/'95 Tech, but I figured it couldn't hurt to put it here also.

Anyway, the sound is similar to a mid-pipe leak and is coming from near the engine bay. It sounds like it is right at the firewall. It is a low tick, not the sharp tick you get from the header gaskets when they go out and is most noticeable when accelerating or going up a hill. The higher the RPMs go, the less I can hear it, but that is probably just due to the exhaust volume. While idling it is still there, but not nearly as noticeable.

It started after installing a Bassani off road X-pipe and removing the AIR/thermactor system, and has lasted through a head/intake swap, two sets of headers, and a Pro-Chamber installation. It is NOT a leak, at least not one that is on the exhaust system. The original heads were on the car when it started, but it now has TFS TW heads with a Cobra intake, along with FMS 1.7 rockers and new lifters. The heads were capped in the back with bolts that threaded into where the thermactor tube used to go. The EGR valve and tube going to the header were also replaced. I have also removed the spout connector to see if it was timing, but it isn't.

Any ideas? This has seriously stumped me, and is getting quite frustrating.
 
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