If the sound lessens as the engine revs up, it could be the harmonic balancer. The rubber around the pulley could be bad. This is quite easy to see from underneath the car.
could be your crank shaft
Check to see if your trunk is open or if the clasp that holds it closed is loose.
sounds like the right front motor mount .i replaced mine and the clunking sound went away
Probably your u joints or the rear end has too much slop.
I have a 2004 c230 and I had a squeaking noise when the engine first started. It was my water pump with uneven wear on the water pump pulley that was causing the sound. I will tell you it could be many different things, but this was my reason.
Most commonly that would be the fly wheels within the transmission. It is advisable to see a mechanic for this problem to avoid a blown transmission.
Buick makes a clunking sound from trans area when accelerating.
starter
If there is a clunking sound when turning a vehicle, the problem might be the ball joints in the front end. The ball joints help to support the A-Frame which helps to keep the wheel on and upright.
movement of suspension components?
A clunking sound when you accelerate or decelerate.
what causes aknocking sound on mazda T35 engine when its running