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Metallic drivetrain noise only upon acceleration

The vehicle in question is a 1998 Ford Ranger XLT with the 3.0L 6cyl engine and an automatic transmission.
I’ve done quite a number of repairs already and am now trying to isolate what might be causing this particular noise.

The noise itself sounds a little bit like a rotational metal on metal sound with a dash of two washers being vibrated together.
It takes no effort at all to reproduce; put the car into drive and step on the gas. The truck will happily move forward, making that drivetrain sound.

Here’s the thing…

  1. If you’re in park and step on the gas, there’s no sound.
  2. If you’re in neutral and step on the gas, there’s no sound.
  3. If you let off the gas and coast, the sound goes away immediately. Step on the gas again, and it comes right back.

To me, this seems to indicate a motor and transmission mount problem. If the engine is idling, then it isn’t spinning the drive axle or rear diff.
Upon acceleration, the driveline might be torquing strangely and this in turn would cause the differential yoke to spin at an odd angle causing the noise. It’d also explain why all the noise ceases when I let up on the gas; there’s no more load on the drivetrain.

I don’t believe it’s the axle or pinion or even wheel bearings, since the car is -quite- quiet once you let up on the gas pedal, but I’m open to suggestions and thoughts.

What do you all think? Tom? Ray? Anybody?

-Bryan H.

It could be as simple as the heat shield on the catalytic converter.

@Triedaq

Hm. Hadn’t considered that, but I would’ve expected it to rattle some while coasting or revving the engine while at a standstill. The exhaust is going to flex no matter what, and if the heat shield was loose and rattling, wouldn’t that induce the noise whilst revving while in neutral and park?

Still, thanks for the idea! I’ll give it a look when I get home from work tonight.

-Bryan

A similar problem, also a Ford Ranger 3.0 engine, turned out to be a break in the crankshaft at the rear main bearing.