Merc S-Class, squeak that doesn't go away when braking??

[The sheriff]

I have a 2000 (X-Reg) Mercedes S280. It drives great, but sometimes at slower speeds, I get a high pitched squeak noise coming from near side. Strangely, it doesn't go away when I press the brake pedal, so that would rule out a brake issue?

If not brakes, anyone any idea what else if could be?

Many thanks

Nick