VW Passat b5.5 engine sounds - please help

[Standish]

hi just had my car serviced, full one, all filters, oils changed and flushed etc

when i first turn the car on it sound like any normal diesel should

however, after about a minute the note of the engine drops and it makes like a low blowing sort of noise. this is only at idle speed and goes when a few revs are put on

no lights on the dash saying danger

any ideas people????