My old c230 had a similar problem, something was beeping/buzzing really loud on random times. Sound went down by time at the end, it was just noticeable if i open the hood....and almost disappeared few weeks later...I think it was the siren for the alarm. I think there is another battery in the alarm siren and it needs to be replaced after so many years...I dont know if it is your scenerio also..good luck.