Valiant Eco tech plus combi bolier?

I recently had my filling loop and valve changed, as i had a problem with the pressure going up and i had to keep empting the bathroom radiator. The plumber/engineer changed these parts and set the gauge at 1 bar ( cold ). after 5 days I heard a loud humming sound from the bolier, after a few mins i switched it off and then on again, and it had gone. 36 hours later i noticed a wet towel under the boiler and underneath ( right hand side) the boiler was silghtly wet. I checked the pressure and it was at 0.8 bar so i guess i have a leak somewhere, any ideas what it could be ?????