Why my ML55 make tapping noise at drivebelt in pace with rev later ok when drive

poss belt tensioner or idler assm

After your vehicle sits overnight or for any extended period of time, the oil settles down into the oil pan at the bottom of the engine. When you first start it, there is residual lubrication on the top end internal parts but the oil pump has not pushed the motor oil up into the lifters, pistons, and valves. Therefore, you hear the tapping sound that then goes away once those parts are fully lubricated. We would have to take a look and listen to see if there is a need to perform any repairs. In the meantime, have a fully synthetic oil put in next time you have an oil change then listen to see it there is an improvement.