Is it normal for a 225 to...

So it only makes it when power is being put to the wheels? Could it be driveline related? How positive are you that it's coming from the engine?

What happens when pressing the accelerator while in neutral? If nothing, then perhaps take a look at driveline. U-joint?

I think U-joint is a very good suggestion and most likely the issue. Have you checked the rearend grease and made sure it is full?