I have a 240 with 425,000 miles. Runs perfectly and no complaints. However there is a moderately loud whine in 3rd gear at 20 to 40 MPH. This disappears in OD at higher speeds. It seems more noticeable this year. What is this and is it something I should worry about?

 

Well, yeah, it is slowly deteriorating as all things mechanical. All you can do is change the fluid, keep up the maintenance until it gets notably worse (undriveable), then a rebuild or a used one. With 425K it doesn't owe you a thing.