KA24 E Valve/rocker noise ~18k mi after rebuild

[driverds20]

1989 240SX Currently at 136k mi.

I have consistently had valve noise right at startup for a couple seconds...used to that for the last few years. But Sunday the valve noise continued a bit longer at idle. I ran it for a minute or so at 2000 RPM to try to bleed the rocker arms, but there was no improvement. Drove a 15 minute trip, and the valves were noisy (at least I could hear them over the road noise and such) whenever the motor was over about 2200 RPM. When I shifted to 5th at 50MPH and the tach went below 2K they quieted down. Did not seem to be affected by throttle position (like spark knock is); temp gauge was ok. Today I drove the car to work, 20 min at just over 70 MPH/3200 RPM, could hear them the whole way. Once off the interstate got quiet again below 2200 RPM. Going home I noticed it seemed to quiet down when revved above about 3500 or so. So the noise is between 2200 and 3500. Power and smoothness seem fine.

All rocker arms were replaced about 18k miles ago, as was rocker shaft on the intake side, to try to get rid of valve noise at startup (which this motor didn't always do - assumed it was a wear issue). Oil pump replaced as part of rebuild 20k miles ago. Ran Pennzoil non-synthetic for 10k miles after rebuild, then Mobil1 synthetic until this last oil change (about 1000 mi ago), when I changed to Mobil Clean5000 10W-30 (non-synthetic) to save money since the car uses about 1 quart in 1000 miles (I know...too soon after the rebuild; a question for another day!).

I'm concerned about this new noise. I could replace the rockers again, but they shouldn't be bad already, should they? Should the rocker shafts always be replaced when you replace the rockers? Could I be getting a lot of oil leakage thru there, and not pumping up the rockers on the exhaust side (rocker shaft still original over there)? Maybe that's why the noise goes away at higher RPM, since the oil pressure is better? Has anyone ever gotten a bad oil pump (I'm sure I bought Nissan OEM)? Do they make high-pressure/high-flow oil pumps I could try?

Any other ideas?

Thanks - Mark

[driverds20]

By the way - oil level is good...I noticed this new noise issue just after topping off. Right now the motor's actually slightly overfilled (about 1/8" over full on the dipstick).