My rear valve cover still makes noise.
I made a video of the noise last summer with my cell phone. You can hear the valve cover noise in the first 2 seconds of this video:
. The reason the sound stops is because as soon as I started moving the mike around, the phone didn't pick up the noise as well.
As far as the head gaskets are concerned, how much are head gaskets? I don't have access to any heads I could swap, so it'd probably have to be you doing the swap.
As soon as it warms up, I'm going to pull my rear valve cover off and check each lifter, each valve spring and the bearings in each of the rocker arms. I suspect I may have a flat spot in one of the rocker arm bearings (secondary to an old valve strike which resulted in the heads being rebuilt). If this fails, than I'm going to do everything I can to get my hands on an oscilloscope to test the injector waveforms during the miss...