Weirdest engine rattle ever, BBC454. Help!

[zwede]

Alright, here's a weird one I've been fighting the last 6 months. Please bear with me, I think I need to put as much detail as possible in here.

Background: BBC454 in a '71 Vette with a 5-speed Tremec behind it.

The engine specs are:

Stock LS-5 block, crank, rods.

Aluminum heads.

Hydraulic roller cam

Headers

EFI

Last fall I had the short block rebuilt due to oil consumption. Found 4 broken piston rings, huuuuuge ring gaps, uneven deck etc. Basically, whoever did the last rebuilt should be shot.

The shortblock was done by a very good machinist who has done engines for people I know for decades. The crank was OK, just needed polishing. Rods OK, resized. Went with Probe Forged pistons and 10.15:1 compression. Decked block so pistons are 0.005" down. With a 0.039" headgasket I have a 0.044" quench.


Re-used everything else (heads, cam, distributor, flywheel, clutch, etc etc). Converted the mechanical clutch to a hydraulic setup while the engine was out.

I assembled the shortblock myself with the help of a guy I trust and has done many of these before. Also, this is not the first engine I've done.

The engine fired right up, no smoke. I've put about 2000 miles on it sofar.

Now the problem: ever since the rebuild it has a very noticeable rattle. The sound is like a heat shield (thin metal) rattling against another metal object. Except I have no heat shields. But that's the sound. A sharp, metallic sound.

Here's were it gets weird: It never does it below 2000 rpm. It also doesn't do it under no load or very light load. I can rev it in neutral, no noise. I can drive it in 1 st, and slowly accelerate, no noise. But under some load it will start rattling at about 2300 rpm. It gets less at 2500, then comes back at 2800.

And to make it weirder: At very high load (close to WOT), it goes away.

Also: The pitch or frequency of the rattle does NOT change with rpm. It always sounds the same. There is no difference when the engine is cold or hot.

I have not been able to pinpoint the noise as it doesn't do it in neutral. It sounds to me that it is coming from the left side, and not from the top of the engine.

What I've done:

Took belts off to disconnect PS, AC and alternator. No change.

Checked exhaust system for clearance. It clears + it's the same system as before the rebuild.

Checked motor mounts.

Checked all bolts/nuts on the engine. No loose bolts found.

Checked bellhousing and tranny bolts. They're tight.

Removed bellhousing splash shield. No change.

Oil pressure is normal. 30 PSI hot idle, 50-60 PSI going down the road. Maxing out at 70 PSI. Oil is Castrol 10W-40.

HEEEELLLPP!!!

[Barbapapa]

Just thinking out loud here, a far fetched idea that comes to mind is the hydraulic clutch line. When you do a big project, anchoring that line may not be something that gets alot of effort and it could be flopping around all forgotten.

[Procision-Auto]

Hmmm...sounds familiar. Had a customer car with similar issues.

Even by rotating the engine and checking clearances with the bellhousing,
nothing could be found until the motor was pulled. Much like you, we
ruled out bearings, driven accessories, etc.

Here is 'his' reply.

But the truth would be told by pulling the motor. So today I took the motor out of the car and when I went to remove the flywheel the bolts were loose and the flywheel is almost cracked in half so most of you were right about the bolts flywheel. so iam going to get a new one and put the motor back in. and hope that’s all that wrong with it I have not driven the car in over two months and that’s all it was I will kick myself in the a$$

Worth a look since you've checked everything else.

[zwede]

Barbapapa: Good catch, but I started out with a hardline and later had a proper braided hose made. There was no change in the rattle.

Procision-auto: I've also thought about the flywheel being loose. I do remember tightening the bolts, as it was hard to keep the crank from turning (engine on stand), so I know they were tightened.... but that doesn't mean they couldn't have come loose... Wondering if I could tell by removing the splash shield and trying to wiggle the flywheel?

[Procision-Auto]

I had a tough time feeling any loose bolts/ or crack without removing
the transmission/engine.

I'd hate to see you pull the tranny for nothing, but I can't give you a
definite yes/no unfortunately.

[zwede]

I had a tough time feeling any loose bolts/ or crack without removing
the transmission/engine.

I'd hate to see you pull the tranny for nothing, but I can't give you a
definite yes/no unfortunately.

As much as I hate having to pull the tranny, I'm running out of ideas so that sucker may come out this weekend...

[bill jones]

-I'd think that if the frequency of the noise doesn't change with rpm then you need to be looking at something that is NOT rotating---for something that is being vibrated by the engine/transmission mass.

-Like the other day I was following a car and the license plate frame was doing a hootchy dance--and I could see it go thru changes as the car changed speeds and drove away from a stop light--sort of like a drummer keeping a beat but just plays the drums louder at times.

-I have used an automotive stethoscope--installed a longer hose and a piece of steel tube at the end of the hose--then hose clamp that in various places under the car---run the hoses into the drivers area-----and listen to the ear pieces while driving.

-It might take a while to find the noise but you already have 6 months futily invested in your efforts.

-You could even do multiple tubes to several places and change the tubes around while driving.

[zwede]

The noise is exactly the same. Both the pitch (i.e. the sound of each tap) as well as the frequency (the number of taps per second).

Trying to localize it with a stethoscope is a good idea.

[Procision-Auto]

The noise is exactly the same. Both the pitch (i.e. the sound of each tap) as well as the frequency (the number of taps per second).

Wow, that's killer! I would have thought you were referring to the pitch
as stated in your initial post.

To have the same number of knocks at any RPM...hmmmm....

Please post the result when you figure it out.

[Tuner]

Yeah, what Bill Jones said^^^. Bill, you’re a mind reader. I wrote this before I saw your post.
Have you used a stethoscope or a piece of hose in your ear and listened around to get closer to the source? A long screwdriver or a stick against the cheekbone right in front of the ear works. You said it doesn’t seem to be on top of the engine, but I’ve heard PCV valves rattle with manifold pressure fluctuations that had everybody thinking something terrible was going on. One time, made an insert that slipped in on a dovetail or tongue & groove to restore the divider in a manifold that had been L-88’ed. It fit tight to start with but it only took a couple of weeks of daily driving to loosen it up and it started rattling (however, with engine speed) and made a mystery noise. Du-uh. I’m with Bill, listen around some more with hose to pinpoint the sound source. This is likely going to be something like a brake line, a piece of linkage or an un-secured body panel with it’s own natural frequency that’s getting excited.

Pictures in this link of “det-cans” used to listen for detonation. Describes the principle Bill is referring to.
http://www.innovatemotorsports.com/foru ... stcount=29

[MileHighMan]

I'm thinking a harmonic balancer, or flywheel issue. Good Luck. Dan.

[airflowdevelop]

Do you have a chassis dyno availible? Especially an above ground?

for a few bucks, it could save a whole lot of trouble!

Dennis

[PackardV8]

Very strange - the noise is load-activated at certain RPMs but doesn't change pitch/frequencey with engine speed. Therefore, it almost can't be a rotating assembly, such as the flywheel or damper. These definitely change with speed. Some sort of sympathetic vibration at work.

1. Something to do with air flow? Like a muffler baffle, choke blade? Secondary air valve?. Heat riser valve? Reving at idle really doesn't make much exhaust volume. Full throttle probably makes enough pressure to pin it over.
2. Does it have an engine-driven fan? Check the shroud, the rivets, the pulley bolts. Some induced vibrations have a natural frequency which wont follow engine speed.
3. Fuel line vibration
4. The T-5 would be a suspect. It is aluminum, it wasn't designed to go behind your BBC. Internal shifter rails? Did you dial indicate the bell housing?

thnx, jv.

[zwede]

Packard: Yeah, it's a weird one alright. I wouldn't be surprised to see a little alien pop it's head out of the throttle body going "TAA-DAAA!!!"

Mufflers: In the back (C3 vettes had the muffler right behind the rear bumper). Noise is from the front. Choke blade: Have none (EFI). Sec air valve: None. Heat riser: None.

Engine driven fan: Nope, electric.

Fuel line: Possible. I've checked and re-checked and the fuel lines haven't changed since the rebuild though. Keep in mind that before the short block rebuild there was no noise.

Same for the Tremec TKO: Same tranny as before the noise, with same flywheel, clutch & bell housing. I have not aligned the bell housing (it is the original GM one), but I've run this bell housing for 10 years and never had any noises before.

Dennis: There used to be a guy with an above ground dynojet, but he sold it. There are some in-ground ones so that could be a possibility.

[Procision-Auto]

Do you have a method to record this noise and upload a file?