I've got a meticulously maintained 1994 E320 Coupe Starmark at 85,000 miles- my pride and joy.
In the morning there is a consistent "metallic tapping"/ "metallic clicking" noise that occurs for about 5-10 seconds upon intial start up of the motor.
It appears to be originating internally from the top of the motor back toward the firewall. The noise has been difficult to source as it only occurs for a short period with a cold motor.
I'd really appreciate help with any ideas on possible causes.
Factory authorized service has already replaced all 24 valves once, all 24 lifters twice, inspected cam, air pump, replaced belt and belt tensioner, exhaust system inspected, cam freeze plugs inspected for leaks,and listened with a stethscope all around externally for sources of the tapping/ clicking noise. Oil pressure observed via instrument cluster, appears to be acting normally.
The motor has had religious 3000 mile or less oil changes since I've owned it at 60K, and I'm running 20-50W dino Penzoil. No engine flushes since I've owned it.