Noise from wheels as if breaks not freeing

Anyone suffered this annoying noise something like "pss,pss,pss" that can only be heard when driving slowly with no aircon blowers on or windows open. First thought was EPB was sticking on but when checking brake discs all 4 were very hot even after a short run so it can not be that.

Main dealer had the car for a day, cleaned off all of the brake dust etc but could find nothing wrong with brakes, noise still persists.

The car is one year old Dynamic with 18k miles.

I think I have noticed that occasionally, it makes you think the front brakes are binding slightly, but I often roll down a very slight incline from stopped and gather speed with no sign of any brake binding, so it may just be the pads touching the discs with no pressure. if I notice it again i'll try lightly touching the brake pedal, see if the noise changes, it maybe nothing to do with brakes, but it is speed related.

Yes I should have mentioned that as soon as you touch the brake the sound goes off but as soon as brake is off the sound returns which points to brakes in my mind. All 4 discs get very hot without any heavy breaking.

Had as similar problem with an Audi TT but this was due to build up of rust around the edge of the discs because there had been no heavy breaking, the cure was to deliberately break heavy!