PLease Help with Engine Sound

I have a 2006 CLK 500 with about 38k miles on it. I just purchased it a couple months ago. It has a small whinining sound coming from under the hood. It kinda sounds like its an electric car. I took it into the dealership and they have had it for 3 weeks now. They have replaced the oil pump twice, 3 oil pressure values, timing chain and the oil pan. (The last 3 because they messed some things up). They said that they were fairly confident that it was the oil pump. The Service Advisor even said that there was a Technical Service Bulletin for this car and the oil pump. After putting the new oil pumps in the sound was still present. Today they compared it to a couple other CLK 500 / CLK 550's that they had in and they all made the sound but even the Service Manager told me that mine was noticeably louder than the others. I guess they are waiting on MB Engineers to get back ahold of them and they are going to go from there.

Does anyone have any ideas? Is a slight humming sound normal from these cars? It seriously sounds kinda like an electric vehicle. It definately gets louder at higher RPM's.

Please let me know. I really want my car back. They have had it longer than I have since I bought it.