Strange noises, need help!

Okay, GRM.

I have had a rather odd noise develop over the last couple months... 

The deets: Mitsu 4G64 SOHC inline 4, 103k miles. 
 

Intermittent rattle (sounds like a noisy lifter) that varies with RPM. No variation of tone, but while idling, the sound will phase in and out. Increase in RPM or load does not affect the sound, just the frequency of ticks.

Usually (but not always) quiets down after reaching operating temp, and sometimes (cold or warm) it will not show up at all.

I removed the exhaust manifold heat shield to no effect, despite the cracks and missing bolts. Exhaust manifold has two broken bolts; one at each end, but doesn't seem to have a leak big enough to make that much noise.

I haven't been able to isolate the sound with a stethoscope, either. 

Last oil change was after the noise appeared, no strange bits or sparkles in the oil. No oil pressure warnings. Mild blue smoke on start up occasionally, not continuous (Probably valve stem seals). Engine is not down on power, no idle issues, no problems with load or RPM...

I'm stumped. Rod knock would have probably ventilated the block by now,  and the engine shows no signs of oil starvation at all. 

Bad lash adjusters? Balance shaft problems (out of phase, etc)? Small furry woodland creatures?

I need the hive's help on this one, cause I'm stumped.

I'll try to get a recording tomorrow, it was quiet this afternoon...

Argh, it sounds terrible

Does load change it?  If not, its likely valve train, maybe hydraulic lifter.  With load, exhaust leak or piston rattle.

My vote is exhaust manifold leak or spun bearing.

You said in your thread the exhaust manifold is not completely fastened properly, right?  If that's the case, I would start there!

Stethescopes are frustrating for me.  Use a piece of 3/8" fuel hose or similar.  Shove one end in your ear and point the hose at suspected things.  When you get right over the noise, it will let you know.

In reply to wvumtnbkr :

Yep. Eight studs, and the forward most and rear most are broken off. I'll get some new studs on order for when I go on vacation after Labor Day.

I used to get freaked out by the 6.8 in my Excursion on a daily basis because it would sound beautiful on cold start and then after a few seconds develop a nasty ticking sound that sounded not unlike a rod bearing.  Once it warmed up, the noise would go away completely.  Even after I decided that it was probably just the standard Triton engine exhaust manifold problems, it would still make my stomach drop in the morning. 

In reply to wae :

Your Triton was piston slap. Many of the mod motors had that issue including the 5.4 in my van. 

That sounds like a lifter to me. 

I don't hear it phasing in and out in the video.  A tick that phases in and out is usually a failing roller that is sometimes high and then sometimes low, so the valve lash changes.

4G64s do not have any rollers in the valvetrain.  Scratch that then.

I have heard of an engine with a wonky valve that had the same effect, the tip had a taper on it, so as the valve rotated in the head, the lash would open up and close.

But, I don't hear it phasing in and out.

What I do hear sounds like a combination of rattly valvetrain and leaky exhaust manifold.