I would take your car in right away to get this looked on. This sounds like the problem with the rubber ring failing around the crankshaft pulley.
I would recommend everyone what has an M112 & M113 engine to get the rubber damper replaced with the modified version. List price is only $70. & with correct tools the job can be done in under 3 hrs on any of the affected engines. Modified part number on the damper is 112-035-09-00 OR 112-035-08-00.
The rubber ring around the crankshaft pulley will show wear/cracks and Will Come LOOSE as the CRANK PULLEY is a Two Piece Pulley!! And When It Does WATCH OUT! The Pulley Will TAKE OUT the Front cover/water pump oil pan and anything else in its way out!
A co-worker of mine had this failure in her '98 E320, and started with a thump thump noise. The noise was the drive belt being shreaded due to the balancer loosing the rubber ring causing the balancer shifting toward the engine block creating the drive belt to shread the inter two ribs.
From my experience on reading posts on this website and talking to my mechanic, there is not always an early warning. The noise you're hearing just might be your "early warning".
Hear are the results of a search under "harmonic balancer":
Here are the results of a search under "crank pulley":