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I finally figured it out!! The cam timing was off!
I put the flywheel pin in to lock at TDC. Went to put the cam blocks on and wasn't even close. I double checked I was at TDC by verifying No.1 piston was at max height. Loosened the sprocket bolts and turned the cams until the tool slid on. Re-tightened sprocket bolts, removed flywheel pin and turned over by hand a couple revs back to TDC. Put the flywheel pin back in and alignment blocks slid on as expected.
I decided to look at everything while I was in there. All the cam journals looked fine. The steel oil sealing rings hadn't grooved out the ledges yet. Installed the new teflon rings and looked at every lifter/roller. All looked great. Re-assembled at the correct timing and the sound is GONE!!
I never had any VANOS or timing related codes or power loss. There was no evidence that the sprockets slipped on the cam faces and all the chain guides looked fine. Both intake and exhaust sprockets were off the same amount. And both impulse sending wheels were lined up with cam correctly, meaning that if I moved away from TDC both tools would have lined up. Either an unqualified tech did not set the timing correctly or the chain jumped a couple teeth. I may never know...
My guess as to what exactly was making the noise is that the DME requests a certain cam degree by activating the VANOS actuators via the solenoids until the cam position sensors read the degree that is requested. If timing is off the VANOS actuators are completely maxed out to reach the requested degree making them rattle. That's my best guess. If anyone has another theory as to why my timing was off and where the noise actually comes from I'd love to hear.
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