1998 Subaru Forester Rattling noise

I'm hearing a low rattling/knocking sound from my 98 Subaru Forester's engine. It doesn't sound like a rod bearing failure--at least not like rod bearings I've heard before (not a consistently loud knock.)

It's loud in the morning when cold, settles down a bit when it warms up, but never really goes away at idle--sounds almost like dieseling from a few feet away, but the knock/rattle is audible up close.

When driving at 2K plus RPM flat and level road, no noise, but when accelerating/decelerating/climbing hills the rattle becomes more pronounced--from the driver's seat it almost sounds like a loose heat shield.

No loss of performance yet, just the noise.

Anyone out there who might have a lead on this?