I have a fairly new 400 in my E350 box truck, and it has a really irritating problem. When cold, it runs on all cylinders, but once it's been loaded down a bit it has a really loud valve tap and drops a cylinder. Could this be a valve sticking in a guide? I've never had this happen with an FE or a 300, but I don't have a whole lot of experience with the 335 series. All ignition components are new, I found the poor running problem the previous owner installed a new engine for- It was the ignition module.
I'm just not looking forward to pulling the valve covers or heads in this thing.

 

It is possible that the guides, or at least one guide, is that tight. But it would have to be extremely tight to overcome the valve spring. I'm wondering if you have an oiling problem. What oil pressure do you see when the engine is warm?

 

About 25-30psi hot idle.

 

If it were me, I would remove the valve covers and start the thing up cold, then let it warm up and let the problem appear, then try to identify exactly which valve seems to have the problem and investigate from there.

Valve springs are pretty strong and I have a hard time envisioning a valve that sticks only after it warms up, I'd question the hydraulic lifters for some reason (assuming the diagnosis is correct).

 

If the valve were sticking in the guide it would have thrown a push rod out and bent it up by now