does it happen just at idle without driving? Ive seen this happen before...car out of alignment and was wearing out the right front tire really badly he actually hit a curb and he was smelling the tire burning up as he was going down the road. Im sure this isnt your case but just throwing that out there.
maybe you have some hoses under the hood burning up or touching something hot. or a pulley thats going bad and maybe burning up your engine belt. or something stuck onto your exhaust pipe burning up.
Mercedes Benz Certified Master Technician