Intermittent But Troubling Noise When Accelerating

I'm getting an odd noise that I've never heard on this or my previous Tacoma. It's sort of a whirring or grinding sound. It occurs only when accelerating - never when idling, coasting, or braking. Tends to be worst when first accelerating (0-20 mph). The noise is also intermittent - it doesn't happen all the time when accelerating and some days it seems to barely happen at all. As best I can tell, it sounds like it's coming from the passenger side of the engine.

No noticeable change in handling or acceleration performance, just noise.

2013 Tacoma automatic 4-cylinder, well-maintained, 115K miles.

Any ideas what might be going on?

 

Driveline rubbing on something only when flexing under acceleration? Is the noise coming from the front or the rear?