The car just recently had a tuneup, and she's running fantastic now, except I've got an engine knock that only presents itself when the engine is warmed up, and the car is in gear. Cold, or in park or neutral, there's no knock and the engine runs like a whispering watch. Warmed up and in drive, however, there's a mild, rhythmic knock that's not particularly loud, but is noticeable.

Has anyone encountered this phenomenon, or have any advice where to start looking?

 

I'd be looking at exhaust clearance around the transmission, I have a knock on hard acceleration from stopped whilst turning right, it's the exhaust touching the trans X member. Check the joint where the exhaust joins the down pipe is tight.

 

Loose or ill seated spark plug.

Sounds bizzare, but I have heard it a few times.

 

Thanks for the input, guys, but the fact that the knock only presents itself when the vehicle is warm and in gear still has me stumped. I feel like if it were an ill-seated plug, it would just make noise without consideration for gear or engine temperature.

 

Can you record it? Maybe enhance the sound with a basic audio editing program? I'm always curious about these things like how do you know for sure? You could become neurotic worrying about every little sound like me.

 

Key words. Tune up. Warm. Load.


That combination suggests a spark knock. Advanced timing.


Just a nutty guess.


Carl

 

I'd be looking for a mechanical knock ie something touching the body work, when the car is warm metal expands and if the clearance is 0.010" cold no noise when warm bingo knock.

As I said I had a knock and it turned out to be exhaust clearance only happened once the car warmed up. Knocks are low frequency and we humans have difficulty in pinpointing these sounds, a good way to locate is to use a bit of hose pipe in your ear and move it around you'll hear the sound getting louder when you're close to its source.

 

Yep,

It has just had a tune up, and now a knock, so I always go back to whatever was disturbed to do said tune up, and ensure ALL is as it was prior to human fiddling.

Usually the culprit is found in that path somewhere.

 

I suppose I could try to capture a recording of it. Maybe con a friend into standing on the brake while I peer around under the hood. Preferably with my mechanic's stethoscope, wherever the hell that ran off to.

Thanks again for all your help. If nothing else, the input has shifted my mind away from the apocalyptic causes and towards the mundane.

 

A strange one I ran into that caused a knock was the catalyst/honeycomb, in the catalytic converter cracked and only knocked when you would back off to shift (5spd) which is indicative of a rod bearing scored. AKA Rod Knock. I thought I was doomed. After checking the oil for metal and checking the oil pressure with a gauge. I noticed the noise had changed and didn't do it at the same time, after listening to it for several days we dropped the pipes off the exhaust, and the noise went away. We could see the honeycomb was cracked and could be physically moved, hence the knock, we replaced the converter and all was good. I'm not sure if the catalyst in the head pipes of the jag are susceptible to breakage or not ? As the car I'm talking about is my 2008 Mustang. But it's not the only converter I've seen crack and rattle like that. Just some thoughts.