6.5 popping back in intake
 

I picked up a 96 K3500 6.5 cheap awhile back and it is making a noise through the intake, kinda sounds like hitting the bottom of an empty barrel. Pulling the upper intake it sounds like it is coming from #1 cylinder by running a piece of hose down each runner and listening. I can also feel air pushing back through that runner with my hand while idling.
I have read up on possibly a non seating intake valve and possibly a broken intake valve spring. The cylinder will hold shop air just fine and is not leaking back into the intake, I also swapped out the intake valve spring with no help at all. All valves appear to be working by watching it run with the valve cover off.
The truck seems to run just fine, no abnormal smoke or misfire that I can tell and no check engine light at this time. I'm kinda running out of ideas here.

Is it out of timing?

Timing would affect only the one cylinder? I did have the cover off of it awhile back and while I was there I lined all the timing marks up and it seemed ok. I never did have the pump timing checked with a scan tool though.

EGR stuck open?

Rocker arm out of line holding intake valve open?

It's an "F" engine so no egr on this one. I've had the rockers off of this motor a few times checking over things and the keepers are all in tact and look normal to me.

The only way for any popping in the intake to happen is from a sticking intake valve in that case,or a bad valve seat holding a valve open-as far as I can think of.

I was under the impression that if the valve seat was holding the valve open at all then the cylinder wouldn't have held pressure when I put shop air in it to change the springs. Is that not always the case? Thanks for all the help so far.

You are correct,if there was any flaws in the valve/seat seal it would have leaked the shop air into the intake runner and out the port.

possable bent or cracked valve. My exhaust valve had a crack in one and a piece missing out of another.

bad injector maybe?