Whine / Squeak - Not Sure

[FunkyFez]

Not posted an 'issue' for a few weeks, but here's the next.
When the car is cold and i slowly move off, i notice a very faint squeak or prolonged 'beep' for a couple of seconds.
Always when the car is cold or when it has been parked for more than 10 minutes.
When the window is down the 'beep' seems to originate from the passenger footwell area (tricky to pinpoint.....again!)
Gear box oil and all the associated messy stuff changed 7k ago.
Not sure if it is even a gearbox sound or a normal MB trait.
Any thoughts?

 

[BlackC55]

Is the exhaust secure?

 

[FunkyFez]

Yeah, it's rock solid.
Doesn't sound like a mechanical sound, it's more like the beep from an answer machine but very quiet. (Sorry can't think of any other way to describe it)