Loose Metal Knocking/Banging Sound during Rough Idle

[RAC68]

Hi All,

For a while now, I have experienced a metallic banging sound coming from low on/in/very-near the engine during a rough idle. This is not like a connecting rod nock but more like something metal hitting against the pan side or under the front engine cover. I have tried to find where this rather loud sound is emanating from without success and find it disappears at higher RPMs when the engine vibrates much less or not at all. My oil pressure is good and I pulled the front cover and replaced the timing chain and hydraulic tensioner a number of years ago. Although one friend suggested it could be a loose oil pump pickup hitting the pan or even the a broken pan separator hitting the side, however, before I go through the difficulties of pulling the pan, I would appreciate any thoughts and/or suggestions as to where this banging could be coming from.

Thanks All,
Ray(64BJ8P1)

 

[steveg]

What about your downpipes or muffler hitting the outrigger or maybe one of their mounts is loose.

 

[RAC68]

Hi Steve,

The down pipes and exhaust system are quite tight with no loose movement ... or movement of any type during the rough Idle. By the way, this has become more of an issue since I installed my Delrin Bushed HD8s and am experiementing with much lower and rougher idel speeds.

Thanks for the thought. Any others would also be appreciated.
Ray(64BJ8P1)

 

[roscoe]

How about checking the front shock mount attach bolts? Perhaps you have some that are not tight. What is the frequency like? Is it slow, medium of fast banging. Are you standing still while you hear it? Do you hear it when moving at the idle rpm? Are your gearbox supports firm. How about engine mounts. This may sound a bit Zen like but think about what could make a noise like this that is external to the engine and sometimes putting a hand on things is just enough to make it stop. The deeper the sound, the heavier the part may be. Can we attach audio files in this forum? Let's hear it.

 

[RAC68]

Hi Jon,

The sound comes only with a rough idle and is irregular as when the engine shakes at a too slow idle. Although the banging may be present during drives, I have not heard any banging then...not even driving on a rough road where something like a loose shock could be noticed. However, I did check your suggestion out and found my shocks to be bolted tight. This afternoon I tried to feel vibration from the banging in the front of the engine and along the sides of the pan but no banging was present. I can't seem to remember anything in the pan or under the front cover that would, if loose, bang against the sides of these components. Has anyone got an idea of what could be loose in there that could make this noise?

Thanks Jon for your suggestions. It is appreciated,
Ray(64BJ8P1)

 

[red57]

Aside from the speed/frequency Jon asked about, is the sound very rhythmic and steady or erratic and hit & miss? If rhythmic, it could be something internal like a bad lifter or piston skirt slap in a bore. If it is more erratic, then I would guess something loose and flapping/banging around.

Do you have a stethoscope? If so you may be able to determine if it is internal or something external as well as narrowing down the location. It can be surprising what you hear with a stethoscope.

Dave

 

[RAC68]

Hi Dave,

The banging is more erratic and seems to happen when the engine shakes as with a too low idle shake. Tomorrow I will be borrowing a friend's Stethoscope and I will be lifting the front of the get further under to reach the pan. I am not comfortable getting under the car when its running and shaking but I don't have the benefit of a lift.

Thanks for your thoughts,
Ray(64BJ8P1)

 

[RAC68]

What's in the pan that, if loos, could bang against the pan?

What's under the front cover that, if loose, could be banging against the cover?
Ray(64BJ8P1)

 

[Michael Oritt]

Ray--

Until about a year back I spent a lot of time with no success trying to find the source of a rattling noise that would seem to only appear at idle. In the course of changing the oil I found chunks of what seemed like pieces of hard plastic in the drain pan and I began to suspect that the timing chain tensioning belt had disintegrated, allowing the chain to rattle against the front cover. Sure enough, the belt was toast and once I replaced it the noise disappeared.

 

[pdplot]

broken motor mount?

 

[RAC68]

pdplot: I checked out the motor mounts and found no issues. I appreciate your suggestion and intend to give the mounts a second look.

Michael: I had a timing chain tensioner issue around 10 years ago and was hoping it was not the problem. At that time, the piston dislodged and fell to the bottom of the cover leaving oil to be freely released. The result, a loss of oil pressure was one indicator. I must admit I was hoping the issue had nothing to do with the front cover as getting to it would require the removal of everything in front (from grill to radiator, fan, and shroud) and the removal of the highly torqued crank bolt. Well, that will be my spring job if that turns out to be the only candidate.

At present, my oil pressure is good and I would have thought the chain would also cause a scrapping sound as well as a possible banging. I will be borrowing a stethoscope to use to hear if both sounds are emanating from the cover.

Thanks again Guys,
Ray(64BJ8P1)​

 

[Michael Oritt]

Ray--

Perhaps a six-cylinder is different but I worked from underneath and did not have to go beyond pulling the radiator. It was, however, a tough job.

 

[CaptRandy]

Second broken motor mount or missing bolt.

 

[gonzo]

RAC 68, you may also consider the Harmonic Balancer / front pulley assembly making the noise. Is it delaminated or loose?

Check for any looseness or play in the pulley, large nut or locking tab. On my BJ7 a low RPM clacking noise from engine was due a broken locking tab which allowed the large pulley nut to loosen causing about 5 Degrees of freeplay in the HB on the crankshaft snout. GONZO

 

[John Turney]

.... Michael: I had a timing chain tensioner issue around 10 years ago and was hoping it was not the problem. At that time, the piston dislodged and fell to the bottom of the cover leaving oil to be freely released. The result, a loss of oil pressure was one indicator. I must admit I was hoping the issue had nothing to do with the front cover as getting to it would require the removal of everything in front (from grill to radiator, fan, and shroud) and the removal of the highly torqued crank bolt. Well, that will be my spring job if that turns out to be the only candidate.

At present, my oil pressure is good and I would have thought the chain would also cause a scrapping sound as well as a possible banging. I will be borrowing a stethoscope to use to hear if both sounds are emanating from the cover.

Thanks again Guys,
Ray(64BJ8P1)​

You should also have a timing chain vibration dampener. A problem with it wouldn't affect the oil pressure.

 

[Patrick67BJ8]

Have you checked the water pump shaft for looseness? Fanblade? Harmonic balancer for worn out woodruff key? Loose exhaust mounts? The drain tubes from the Intake manifolds can make a rattling sound at idle. Check them to see that they are properly in their brackets next to the oil pan and have their rubber grommets in place.

 

[vette]

Ray I was also going to suggest a stethoscope so be careful. If it is one designed for automotive use they are very, very sensitive and I believe could quite easily hurt your ears. Mine is so sensitive that I only use it in one ear. Aside from that they are quite handy and can bring in many variations of noises that are quite useful for troubleshooting.
So another point is that you said earlier that the noise seemed to start about the time you installed the HD8s. You know the saying, 'go back to the last thing you did'. Could it maybe be the heat shield or something hitting the tinny shield. Also after all these years with the HD8s I finally said the 'ell with the slow idle, I'm idling at 900 to 1000 rpms now. Good Luck.

 

[Bob_Spidell]

This outrageously expensive stethoscope works reasonably well (don't forget your 20% off coupon):

https://www.harborfreight.com/mechanics-stethoscope-63691.html

 

[steveg]

I had a dry rocker arm that made an off-and-on very loud rattling or rapping sound.

 

[roscoe]

Unless I am mistaken the rubber bits Michael refers to are from the rubber "damper" ring which I do not believe is used on the six cylinder cars.