[Wraithman]

Should be easier to locate with a stethoscope or similar .

 

[Wraithman]

If you can find a shop or friend with "chassis ears" you may be in luck. They are very helpful

 

[Wraithman]

I would bet the timing is off causing valve chatter upon acceleration. Check your timing before you do anything BTW chassis ears are a series of microphones and can locate these issues.

 

[Wraithman]

Pull the rockerarm/valve cover off the suspected side. Check the springs on the rocker arm shaft. When broken, they can cause clatter with the rocker arm nearest the spring.

 

[Wraithman]

Have you checked the viscous fan clutch?. The noise seems external from the motor. So the noise appears only in gear and not in park and revved? You'll find it.

 

[Wraithman]

It's also possible to have a broken baffle in a muffler housing that would rattle with higher flow when accelerated, although I think the noise would be a bit muted. You have to consider everything. A friend can help localize the noise.

 

[Wraithman]

I was going to mention the flexplate/starter ring-to-torque converter bolts as well like Jim mentioned since this exact thing happened to me on the highway with my Suburban a long time ago. After removing the inspection plate a beat up short bolt dropped out. It will make quite a racquet and shock the hell out of you when it happens and it has no distinct pattern to it.

 

[Wraithman]

There is an inspection plate on the bellhousing. Once you're under there, it will be obvious.

 

[Wraithman]

Take another look at the 2 bolts on top of the compressor...not tied in?

 

[Wraithman]

Your smartphone camera is your friend. Also, witness marks, especially on rotating components that come apart...paint pen is helpfull
You will have to support the motor via the sump pan which is robust, blocks of wood will work well here. The torque conver houses 2 opposing fans and holds lots of fluid. When separating the gearbox from the engine, thread a heavy piece of baling wire across the torque converter and fasten so the TC will not slip off....they are heavy with fluid!

 

[Wraithman]

BTW disconnect battery 1st, since the starter has to be removed. Endoscope plugged into your smartphone may be useful for deep inspections.