Some questions:
- Are the lifters noisy at start up only, or all the time?
- What is the oil pressure when it does this?
- Got any cam specs? And an engine size? For 500+ HP, even a stroker is gonna have a larger cam and that can open up a few cans of worms.
- What rockers are being used? If the lift is high, then there are other possible sources.
- New engine? What break-in oil was used?
- Any chance this is exhaust impulse noise in the headers?