[Stocker]

This is probably totally off base but it popped into my mind so I'll mention it. I know you said it sounds like the noise is coming from the rear of the truck. But sound can travel and be tricky to locate. so bear with me.

It only happens under acceleration. Motor mounts will normally keep the engine in position, but a bad / broken / loose motor mount can allow the engine to rotate upwards on one side. Not sure what noises might be caused, but one possibility is the fan contacting the shroud. Should be easy to see evidence of that on the shroud.

That's all I have....