I inherited this ST with a Chinese Landsail tyre on each corner and the road noise is quite high.  It seems to be a given that the Focus transmits road noise to the cabin.  If you have an ST and have found quiet tyres, I would be interested to know which brand.   The snag with the quoted db levels is that they are measured externally and probably give varying results for different cars. 

I don't know why anyone would put budget tyres on a focus ST but I will put money on these being the source of your noise.  Goodyear F1, Michelin, bridgestone are good premium brands, good mid range tyres like Fulda, Avon, Yokohama tyres are worth a look.  Personally I would avoid any budget tyres. 

I would say the tyres are the fault , as Jonathon says why put budget tyres on an ST, in fact why budget tyres full stop. Commonly know as ditch finders

Thanks for your comments.  Although there is a fair amount of tread left, I shall be changing them come the spring, whenever that is!

Splashed out (big splash) on a set of Vredestein Quatrac 5 which have reduced the noise slightly and the car feels more firmly in contact with the road.  Interesting feature - they have a ridge that protrudes slightly wider than the wheel rim to protect it.  Took the opportunity to have the steering geometry set up on the latest all-bells-and-whistles equipment; the machine prints out a sort of block graph with the readings - red for adjustment required and green for OK.  After that lot, no more champagne breakfasts for a month or two!