350 V8 pushrod taps I tighten it stops, but later it taps again?

I recently put new head gaskets on a Chevy 350 V8. I thought I had valves all ajusted ok, but after running the engine about 25 miles at about 65 miles per hour it started tapping on one lifter on the driver's side furthest back next to the firewall. I take the last out of it again and go about a 1/2 turn more, put the cover back on and run the engine a few miles and it starts tapping again. Is something collapsing? Is something not holding pressure? What could it be and what options do I have with out taking the intake manifold off etc? The heads were redone for valves and springs a couple years ago.

I went thru this with new lifters that wouldnt pump up, so check that out first since its only one tapping.

If you just keep tighening the rocker, soon you will have the lifter bottomed out and could easily do some damage to the valve train and possibly the rest of the motor. I would suggest getting the offending lifter replaced soon. JMO

Did you take out the lifters at all when you did the head gaskets? What did you touch at all that had to do with the cam and lifters? It sounds like either a lifter or cam lobe going away, either one being not good at all. Did you use new rocker arm nuts? If they are the stock type, they aren't made to use over and over, and it might be that the one in question is just not holding any more. (I doubt that is it) If this problem came on suddenly after you did the engine work, it almost has to be related to something that was done. No problems like this with this engine before along these lines? John

Do you have screw-in rocker studs?
And perhaps stronger than stock springs?
If you're running press-in rocker studs you may have one backing out.

Another possibility is if you have self locking ball nuts one of them may not be locking/holding as it should.

are you sure the locks are holding and that the nut isnt backing off??....could be that simple