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.

Edited March 9, 201511 yr by DBCooper

Not a rod knock.  Those are consistent at all rpms.  Sounds familiar though...let me think about it.

I agree.  I'm inclined to think it jumped a cog on the timing belt and bent a valve, but, like I said, I'm not a Subaru specialist.

I wonder if it is the bearing failing in the cogged-tooth timing belt idler?

Does your engine have a rev limiter?? If so, then not over revved.

 

Sounds more like something outside the engine, like piece of metal that is loose and rattling, or tapping against something. Might be good to use one of those engine stethoscopes (similar to like a family doctor uses) to probe around the motor to try to isolate the source of the sound. Motor sounds good, and revs up easily, so those are positive signs, that the motor may not be damaged.

 

A WRX motor in a 914 has to be a good ride! I bet the power and handling makes for a good combination.

That was my first thought too, some kind of harmonic, but haven't found anything.   It was hard to start right after the bump and doesn't run as well now as it did, both those things pointing to the valve timing being off or a bent valve.  But to me that sound is still somewhere nearer the surface of the engine than a rod, maybe even external but somehow connected to the rotating assemblies, I just don't know have enough Subaru experience to recognize it.

 

Yes, it's a really lightweight car, which makes it makes it a whole lot of fun.  When it runs.

I can't see embedded videos on the mobile. Can you post a link to the vid?

Sure, here: www.youtube.com/watch?v=Axs1QF_Wxzs

Edited March 9, 201511 yr by DBCooper

Ooohhhhh that is nasty and sharp. I'm inclined to think wrist pin or rod bearing. Maybe not the bearing making noise, but the rod could be hitting the block.

 

It sounds like its running rough when you open the throttle part way. But it idles too well to have jumped timing or bent valves. You'll only hear bent valves one time anyway. Maybe twice if it skips a little more, but then it will be so far out of time it'll hardly run at all.

 

Maybe its camera trickery... I swear I see sparks or arcing of some kind near the alternator when you first start revving it up.

Here's a similar noise in an STI :

 

 

Having a little too much fun with the go pedal.

Here's a similar noise in an STI :

 

 

Having a little too much fun with the go pedal.

Noise sounds more then similar, sounds the same to me. Video write-ups thought maybe engine got starved for oil, so spun a rod bearing.

Good explanation of what causes these noises. 

 

THIS is a rod knock! https://www.youtube.com/watch?v=WS37L1G8lnA

 

I have come to the conclusion that what you're hearing is detonation.  What fuel are you running? Any mods? BOV? Tune? etc? 

If that's detonation, that's like a diesel running on ether kind of detonation. Just too loud to be detonation IMO. Detonation like that on a gas engine is going to blow a piston out of the block in a heartbeat.