Horrible knocking/tapping noise at low RPM, please help me diagnose
 

Hey guys,

This is my first post, but I have used this board quite a bit to help with my 1969 GTO. The problem is my beloved 69 GTO has developed a terrible knocking or pinging noise that comes in at about 1300-1500 RPM then goes away at higher RPMs. This is a link to a video of the noise. I have the window down so you can hear it:

http://www.youtube.com/watch?v=iasYf...ature=youtu.be

Things I have tried that have no effect:

Decreased timing to 5 degrees
New module and coil
Disconnected vacuum advance
Adjusted advance curve to come in late (i.e., heavy springs)
New edelbrock victor jr. water pump
New timing chain and cover
New fuel pump
Changed to heavier oil (10W-40)

At 5 degrees, she wants to stall in gear, and runs like molasses. I am hopeful someone will say, "just ignore it," but all of the research I have done seems to indicate some sort of rod knock or piston slap.

The car has the original block, but I am unsure whether the internals are original. It is an automatic with a TH400 (I don't know if the tranny is original.) I also installed a hi-torq starter recently.

What is this and how do I fix it? All help (including admonitions and sarcasm) will be greatly appreciated! :)

doese it only happen when you drive? how about in park and you rev it to 1300-1500? did you check the flex plate? could have a crack on it.

Could it be bad fuel? What octane fuel do you use?

Greg, it only happens in drive under load. If I accelerate slowly, it is less noticeable. I have not checked the flex plate, and will do so.

Heups, i have 89 octane in it. Could it be that simple? What do you use?

Sounds a bit fast to be rod knock at that rpm. I would check motor mounts make sure your engine is not moving and making the fan blades or exhaust hit something a good way to check is jump in put it in drive with your foot on the brake give it a little gas see if the engine is flopping around like a fish in there. Or if it is internal get a mechanics stethoscope makes it easier to pinpoint an issue.

Nar, good thought. I'll take a look tonight.

I vote fan blade or flexplate

1st I'd try some higher octane gas.

Try taking it up to around 2500 RPM or so on a road, then give it the gas.
See if it makes that noise under load at a higher RPM.

Check to see if the fan is hitting the shroud?

:)

Does the sound change when the engine is ice cold versus warmed up?
Piston slap is pretty distinct.
Cheap timing chain slap?
How many miles on the engine and do you know anything about the internals?
A wrist pin coming out would be bad but you may find something premature in the oil or the oil filter… If something on top of the Pistons has come loose you might check all your spark plugs.
Torque converter or flexplate bolts transmit sound all over the darn place when they start to come out and tap on things. It is easier to hear them though when you're underneath the car.

Hopefully it is not catastrophic! Let us know what you find!

That sounds pretty high speed. I'm going for fan blade… If you really want to be sure, get into it really hard, power brake it, continue that until the fan comes apart and destroys your radiator and then you will know for sure.
( just kidding. Don't actually do that… LOL )

The fan is not centered in the shroud (it is low). I'll try running it without the fan for about 20 ft and see what happens.

So you're saying wait for an explosion then rebuild the engine? Got it. :thumbup:

My 69 started that not long after I put in the horrid Taiwanese core support. The bottom of the shroud is about a millimeter away from the fan...

The engine sounds the same warmed up or cold.
Hopefully it is not timing chain slap, as I just replaced it. I used a Compcams double roller.
I don't know how many miles are on the engine. The PO was a dealership, the PPO was some kid in Michigan who left one of his tickets for doing a burnout in the glove box.
When you say "check your spark plugs", what am I looking for?
Also, do torque converter or flex plate bolts make that much noise when loose? Would loose bolts be consistent with the intermittent sound only occurring at a specific RPM range, under load, and mid to hard acceleration?

Thanks for all the help.

Assuming this is the problem, what was your solution? Electric fan, smaller fan? RPM dependent earplugs?

With the video it sounds like five different things are passing one point and striking them, hence the high-frequency sound. I.e. Fan blades striking the shroud as the engine rocks.
If it was one bolt or one thing going around it would sound slower. Internal noises generally have a lower frequency "thunk".
It doesn't sound that internal to me. Of course it is a video from iPhone then listened through an iPad… LOL if something is meeting the tar out of your spark plugs you have much more issues. I'm voting no for that.
Torque converter bolts or flexplate bolts can make all kinds of noises because they have the megaphone of the transmission housing… But again, your sounds more high frequency.

I didn't have anything to lose because the core support was so lousy, I fixed it with a 2 x 4 and a hammer.
You will see marks on your shroud where the fan is hitting it. If you don't, you can line the inside of the shroud lip with masking tape and then see where the fan tears ( or doesn't) the tape off.

Here's a dumb question, and I don't mean to insult you, but if you changed the timing chain are you positively sure the cam retention plate is in place?

bad motor mount, causing engine to lift, fan hitting shroud maybe?