Have been lurking and not done many posts, but I need some help now. Here goes:

I am at a loss regarding a whirring noise coming from the passenger foot well of my w202 C240T.
Driving slowly forward, I hear a distinct whirring which disappears with some speed as it gets drowned out by other mechanical and road noise. It is not an engine noise because at idle and when stopped, the whirring is not there.
Here is what I have done up till now.
1. Checked closely under the vehicle to see if anything was hanging or untied: Nothing
2. Changed front wheel bearings. No change
3. Cleaned and checked wheel hubs: Nothing
4. Changed front and rear brake rotors: No change
5. Checked closely brake callipers: Nothing
6. Changed front and rear brake pads: No change
7. Flushed and bled the brakes: No change apart from a nice firm pedal now
8. Checked steering box holding bolts and controlled torque level: Nothing
9. ATF and filter have been changed recently during last service
10. Rear diff oil changed also during last service
11. Transmission shaft and central bearing checked

After all the above work, I took the car to my trusty mechanic to he could check what I had done and get his ideas on the reason for the noise. He checked everything again and didn’t find anything. His only comment was that the noise could be in the tranny and I shouldn’t be worried. Well I am as the vehicle is my trusty steed, especially in winter as my other car a C43 will be stored away and kept off the cold and salty roads.
So, does anybody have any idea what is wrong, what could be checked over and the cause for the whirring? Greatly appreciate any ideas and comments. Thanks