i have lately bought an sl500 2005 with 125000 kms on it. the care drives like a charm. a week ago i took it for a brake pad change and wheel alignment. after the service was done. i started hearing a knocking noise for a couple of seconds after starting the car. then no knocking is heard. then two days later the sound is still present but know i start hearing it a while after driving the car, the sound is intermittent, it looks like it's independent from the road bumps. it's like one time it's five or six consecutive knocks, some times it's two or one. i got a continous knocking when i tried to inspect the sound i was going uphill very slow like 4 or 5 mph. the car has had a motor mounts change a month ago.the car has been at two mechanics to date they where both unable to determin the sound cause and location. yesterday i had that idea to open the driver door when driving slow and try to hear the sound. it looks like it became louder and coming from the front left side. we removed the the fender internal covers and tried to locate the sound with no sucess. i wasn't able to be sure if it was related to the ABC dampener that is located in that area. the valve bloc is not leaking, there is a minor sweating in one of the hoses. when the sound is repeated several consecutive times it looks like it's a piston that is hitting hard, but when it hapens once or twice it looks like it's somthing metalic hitting the car body.
today i pumped up the car in the morning and drove to work with no audible sound. i lowered the car once and i started hearing the sound again, i pumped up the car thinking that the sound should stop (same as the morning status) the sound remained, so it doesn't look related to the lowering and hightening of the car.
this issue is causing me nightmares. any help is ,much appreciated.

 

any suggestion?

 

Your description is too vague. Does the sound only happen when moving, or also when stopped? Does it do it in every gear including neutral & reverse?

 

Cracked subframe bushings?