Noisy Rockers

I have a 390 in a Mustang and after changing the lifters(anti-pump up),pushrods and rockers (adjustable Crane) I still have tappet noise at 1500RPM.There is no noise at any other RPM just 1500RPM. The motor was rebuilt last year (done 2000Km`s)and the noise has been there since the rebuild at 1500RPM.The new motor included a new cam and intake manifold, the heads were also re done with new springs and valves/seats. We adjusted the rockers down 30 thou after intiall contact using dial gauge.
Has anyone had this problem before?
Cheers