After a recent 400 mile trip in my 2012 C63 estate I'm experiencing a cyclic (wow,wow,wow) noise, it seems to be coming from the front, it can only be heard on smooth roads at 20 to 40 mph and then disappears into general road noise, tyres are conti sport 3's on 19's plenty of tread, in balance and not showing any unusual wear pattern, I've swopped them left to right and I've tried swerving left to right to eliminate wheel bearing noise and doesn't make any difference....this has only been noticeable after the long trip and can't be heard from the outside, before it goes back to the dealers any ideas anyone?