Mystery Noise from ZD326 Rear End When Accelerating

[Suburban Plowboy]

Today I noticed that my ZD326 seems to make noise when I accelerate using the motion handles. It appears to come from the rear end. It's kind of like a dry door hinge, only, of course, louder. It goes away when I'm not accelerating from a stop.

Any idea what it is? Is the mower about to blow up?