I have a very pronounced vibration when cranking the car. It appears to run through the steering column but could possibly be under the hood. It lasts about 5 seconds. When it first appeared it was only on a cold crank but now it occurs on about every crank. That is the only time it's heard or felt. Mercedes dealer has had it for 8 days with no luck. Recent repairs have been the transmission mounts, flex disks, and starter. The mechanics seem baffled. They've used a "stethoscope" to try and pinpoint it but to no avail. Does anyone have suggestions?