3.slow valve chatter --- bad
 

I have a low mileage 3.0 that was put into the truck by the PO. The engine has some Downey headers and whoever installed them was so incompetent it is not even funny, both headers leaked badly. Anyway, I have replaced the manifold gaskets which are doing great. But now I have bad valve chatter... Curious on technique on this. I was simply going to adjust them but not sure if it will do the trick. I have been hearing about some shims that are necessary??

Leaking exhaust manifolds/headers at the head will burn up valves. I hate to say it but I think you need more than just an adjustment.

May as well pull the valve covers and check the clearance. Weak valve springs could also cause a sort of clatter as well. Does this clatter go away when you touch the throttle at all or is still present? Is the clatter more pronounced when the engine is cold or hot? The valve clearance actually increases as the engine heats up due to the head of the valve getting hot and expanding. The 3.0 fuel injectors do make a fair amount of clatter as well but when you bump the accelerator they quiet up as more fuel is injected. Typically a valve will tighten with age and get really quiet eventually hanging open and causing problems (burnt valve).

On flat drives even at speed, it is fine it is on the big sweeping climbs that we have out here, uphill undred throttle that it is most noticable. About 2000-3500 rpms, if the truck is underload and you give it substantially more gas then it starts chattering away. My technique right now is avoid it with my foot...

If this were the city or I live in a relatively flat area I would probably hardly notice it. I also went through yesterday and worked on checking vacuum leaks.. I noticed some rotting hoses so fixed those and seems to be running better than ever and the chatter is reduced as well. Interesting that vaccuum leaks are affecting it...

Hope it helps... I spoke with my friend who is a Toyota trained tech, he says do a compression check first and then start there...

Here's a random weird thought for you too. I thought the valvetrain clatter was getting worse and worse on my truck. Then I discovered the rubber seal where the steering column goes through the firewall was all worn out. A ton of noise was coming through there. I put in a new seal and I don't hear any more ticking. Just something to check if you're hearing a lot of clatter when driving.

Vacuum leak = lean condition = detonation (aka...pre-ignition)

you may want to slightly retard the timing and recheck for the clattering. I would recommend a tank of super to help eliminate the possibility of detonation. Throttle position has absolutely no influence on valve clatter, however detonation usually has everything to due with throttle position while under load.

Another thought is to start the vehicle up give it 20-30 sec to run then go drive it. While its still cold get it into a situation that would normally cause excessive clatter. If you have no clatter when its cold but the clatter comes on when it reaches operating temp then I would again point towards pre-ignition. Detonation sounds like a "paint can rattle ball" to me if that helps you at all.

my truck does that too, i find it only happens when im decelerating in 2nd gear and the truck is going too slow to be in second and its running at like 500 rpm or less and i try to accelerate. i will hear it once the motor struggles to regain speed, but thats about it...

If I were you I'd find a good running 3.slow somewhere and listen to that before tearing into yours.
The 3VZE's injectors are very loud and sound a lot like valve chatter.
This is normal.