Pop the VC and check all you shims and guides? While its off check your rockers+lobes for scufing and even pop off the spray bars and clean them out with some carb cleaner.
The motor dosen't have lifters it has Hydrolic valve lash adjusters(HVAL'S). Sometimes they go bad and can't get oil pumped into them

Before trying marvel mo(I heard its ng anyway) I'd try an other engine flush. What did you use for the flush?
98sentraSE Turbo