Some crocodiles have a real problem with the way the gears mesh and will sound like a helicopter. I had to replace the bogies on my 8852 with those from a scrap 8856 to fix the problem.
The bogies can be extremely 'notchy' in operation when turned by hand with the upper main gear - nothing I tried made an improvement but one day I will investigate further by looking at the gears etc under the microscope.

Even a perfectly functioning z gauge loco can be pretty noisy. The surface the track is laid on can make a huge difference to the way the locos sound!