cobrajet

Hi can anybody help me I have a 1999 c-280 sport I have 135000 kms maintained perfectly when engine is cold and start the car this a engine knock as the engine idles and warms up and slowly the noise dissipates as enigine warms up to operating temp don't quite no what it is lifters are new and replaced I was told it might be a noise coming out of top of piston due to carbon build up mostly drive in city highway once in a while oil is change every 6-7000 km engine runs great lots of power does not burn oil only when engine is cold it knocks and slowly goes away cannot figure it out can or any one have any solutions thanks!