My 2005 C50 limited has experienced a cold engine knock since I purchased it at 2,500 miles. It gradually diminishes as the bike warms up, and the knock seems to come from the bottom end area. It has gradually become more pronounced as the miles have accumulated (now 15,000). The bike has been well maintained and has been run on Amzoil synthetic since 5,000 miles and the original owner had the initial service done on schedule. Noise more pronounced under load. I seem to recall that some owners had experienced this noise on C50's also. Does anyone know what the source of the noise is? Appreciate any input.