Very slow tap on guardrail, 5 mph max, but maybe over-revved the motor with foot on brake while sliding for a while on a slick road. Now it has an inconsistent knock or tapping when revved It doesn't sound like a rod to me, but I'm not a Subaru specialist. Could you give it a listen? On YouTube, here:
The knock, really a rattle or tap, starts at about 20 seconds. It's not the timing belt, removed the covers and pushed on the tensioner, no change.
Forgot to mention that it's a WRX engine in a Porsche 914, but otherwise stock. Thanks.