This may sound strange, but it’s worth a try.
My wife’s 1993 300E had a strange chirping, almost cricket like sound coming from it regularly. It was hard to tell where it was coming from and it puzzled me for a long time until I slammed the trunk one day. The noise was actually created by the trunk closing mechanism, a spring as I remember. It was loud and weird. It sounds like your noise is coming from under the hood but I thought it was worth passing on the information.
Good Luck