HELP!!!!...sbc engine noise

[SR71]

a friend has a 406 sbc that we put together many years ago. it has been hanging on a stand sealed up for the past 10 yrs. the valves were not adjusted so there was no pressure on the valvetrain and he rotated it every few months to keep it from sticking.

well, he finally got a car to put it in and we installed it. we prelubed her, adjusted the valves and fired it up. we then broke the camshaft in (hyd flat tappet) per the mfr instructions. things went great. good oil pressure and temp. I idled the engine down to about 950 rpms and she sat there loping and sounding badass. and then........



Rich pulled her in gear. when the converter drug the idle down a little bit, she went to tapping pretty darn loud. the noise seems to be coming from the right hand bank. it sounded like a lifter waaay out of adjustment, so I ran the valves to zero lash with the engine running. sounded great again until we pulled it in gear. the noise goes away when you idle the engine up, but comes back when it chugs down.

the tap seems to occur at about half of engine speed so we think it is valvetrain related. we checked the valvetrain for any mechanical interference and found none. then we installed a new set of lifters and broke them in again. we still have the same problem. we even swapped the roller rockers out for pivot ball long slots and the tap is still there. the distributor was the next thing that I checked. no dice.

we also checked the convertor bolts, flywheel bolts and then the headers and oil pan for any contact. nada.

the engine is hanging on the stand again with the pans and heads off. we cannot find anything wrong. no witness marks on any of the parts either. the cam and lifters look new.

this thing ran like that raped ape of lore. but the tapping noise was obscene at idle in gear. and this was while holding 35psi of hot oil pressure. the heads are 210 afr's w/2.08 valves and there is no p to v contact.

I am all out of ideas. any help is appreciated.