I've been having a problem with a bad metal-metal pounding coming from my engine. This started back in January, and after rebuilding and checking the engine over (with a few addons) I fired it up again this past weekend, to hear it have the same problem. Its not the heat shield or "normal honda ticking". It sounds like a hydraulic lifter thats gone bad, but obviously, they don't have lifters. Its not the LMA's either... already checked. It happens throughout the entire usable rpm range (up to the 4500rpm wall for engine break-in). Something else I've noticed, probably related to the problem, is a surge in vacuum. I didn't pay much attention to it at first, because I noticed I was getting boost into the engine (and partial boost) around 3k, instead of the usual 4k. What I don't like is that the vacuum shoots up to around 10inHG when I'm cruising, where it was at 20inHG before this all started happening. Also, instead of a smooth climb, before building into boost... the gage shoots. The engine then falls flat on its face at 4k rpm, with very little accelleration or pull.
Does anyone have an idea what could cause this? I've heard everything from cracked head (which was NDI'd and found to have no cracks) to a broken valve spring (none, but one was .015" under stock height) to a bad cam (which isn't warped, but I have no real way of checking for flat spots and such). Any help I could get on this would be much appreciated.