Body shell resonance

A couple of my Marklin electric locos make what sounds like gear train noise, but after a process of elimination (running the chasis around the layout without the body shell) it turned out to be body shell resonance - they've all got plastic bodies and the noise isn't made by the metal-bodied locos. I tried glueing a bit of foam inside the shell of one as a test, but it made no difference.

Has anyone else encountered this phenomenon? Does it happen with MTL diesels? Is there anything else I could try?

Given that the sound is not unlike the real thing I'm inclined to pretend that they've had a realistic sound card installed