Lots of things could be wrong, but I'll try to help narrow it down.

Did you hear the "strange sound" after you shut the engine off? If so, I'm guessing that the noise you heard was the traction system charging pump. It's not uncommon for the relay to stick closed, causing the pump to run continuously until the pump dies, or the battery goes dead. This will usually store fault code C1401 in ESP.

I'd recommend charging the battery(with the ground dis-connected), and checking to see if the pump starts running when you re-connect it. If it does, dis-connect the battery again and remove relay k25. With the relay removed you can drive it without risking charging pump damage, assuming it hasn't been damaged already. Either way, you need to have it checked by someone with experience on MB traction systems. Let us know what happens.