I presume you, (punkalouver), know the sound of the Airbus FBW yellow hyd pump, which runs during single engine taxi?
The 'hacksaw' noise is the hydraulic PTU transferring power between the Green and Yellow hydraulic systems, as they pressurise or depressurise when the engines are started or shut down.
The sound, also known as a 'barking dog in the hold' is worse when the hydraulic accumulators are not fully gas charged. (The PTU is also automatically tested during engine start).
PS to the OP; the cabin recirc fans can be switched off to see if they are the cause, (as can the avionics fans, but I doubt it is those).