Quiet Humming Noise After Taking Foot Off Accelerator, What Could It Be ?

I've been having problems with my maf sensor, replaced that but then diagnostics tell us a fault in wiring to the maf, circuit A open. Wires looked damaged so we cut the ends off and connected them back up. Now my car accelerates again from a cold engine ( wasn't accelerating previously until engine warmed)

For the past few weeks, before we even repaired the maf sensor wiring, I have been hearing a quiet humming noise after I take my foot off the accelerator, it seems loudest when engine is cold and I'm reving the car at idle. Can't hear it when driving, I think it's only when idling and I rev it . when my foot is on the accelerator I cannot hear the humming, I only hear it when taking my foot off the gas.

I mentioned it to my mechanic friend who much to my frustration says he cannot hear the noise, but maybe that's because the engine is usually hot when I go to see him.

Car is a mk2 2006 diesel focus 1.8 sport

so, any ideas what the noise could be?

Thanks everyone !