engine knock problem

I have a 1996 mustang with a V6 motor. The motor started with a slight ticking noise, the noise turned into a knock which comes and goes, it does not knock all the time. The motor does not use any oil the oil pressure has not dropped and it seems to have lost no power. I have took out lifters inspected them re-installed they all seem good but the noise has'nt stopped. there is 85000 miles on motor. Can someone please help. [email protected]