how can i solve a whuring noise on my ford escort 1.8 tdi finesse?

i have fitted new wheels on my escort.,and i have developed a whuring noise when i slow down.The wheels are 16" alloys with 205/50/16 tyres on them.the original alloys i took off were standard 15" alloys(ford).The car apart from this new noise drives really well.am i just imagining this sound????