Strange engine noise

[SharkPilot]

I need some advice from the more experienced people(than me) here. This is going to be a bit long winded but I'm trying to be thorough.
On two occasions my engine has made a rattling sound. The first time was about five weeks ago and it started just as I was merging onto the freeway. It has the rhythm of a valve or lifter and the engine had a miss for just a few seconds then returned to running normally, except for the rattle. The nature of the rattle changed a bit and then it quit. It was so brief I even wondered if it was my car or not, except for the momentary misfire.
After that I removed all eight plugs, thinking something may have gone through the engine. All eight looked perfect and the engine continued to run great.
Five weeks and about 750 miles go by and yesterday it happened again. I exited the freeway and heard a rattle. The engine misfired a couple of times and when stopping at a signal was rough and the idle dropped down to 300-400 rpm. I feathered the throttle and it came back to normal and acted fine the rest of the way to Mom's house, about 1/4 mile. Once there I opened the hood and listened to each bank. The noise seemed to be coming from each side evenly. The idle was perfectly smooth and even the exhaust at the rear sounded totally normal.
The car sat for six hours and started fine to go home. The rattle was still there and I noticed that the nature of the sound varied with engine load. It sounded harsher under light acceleration. I was only going 15-25 mph through the neighborhood. When I turned out on to the road the noise altered again and then disappeared entirely. The engine ran perfectly for the 12 miles home and also ran great today.
I was thinking a valve lifter until I noticed how the nature of the rattle varies with engine load. When coming to a stop, when the clutch is depressed just before rolling to a complete stop it almost went away only to return when the engine got to idle speed.
I do have a rather noisy flywheel that rattles on just about every shutdown. I run Mobil1 oil (10W-30) and change it every 4-5000 miles. The car has about 55,000 miles on the stock engine.
This is more than a bit $cary and I wonder if anyone else has experienced this with their engines? Does anyone have any idea about what I could be checking for?
Thanks for the help,
SharkPilot

[A26B]

The rattle, frequency of the noise and the associated miss all point to a collapsed lifter, due to a intermittently leaking ball check (yellow) or worn plunger.

[gbrtng]

You said: "I was thinking a valve lifter until I noticed how the nature of the rattle varies with engine load."

which leads me to think the ignition timing is wandering around and going to full advance causing extreme detonation occasionally. You need to find a scan tool and watch some of the basic ECM inputs and outputs. You might even have an ECM that's going south, but further diagnosis is required before you start throwing parts at it.

[scottfab]

I agree gathering scan data is likely to be the best bet but....
does the noise change with eng temp? That is, is the noise louder
at startup and quiets down with the occasional outburst of noise?

Does it sound like this:
http://scotfab.tripod.com/piston_slap1.wav

[SharkPilot]

Scott, I can't access that link. It says it's not available for download.
The noise was the same hot and cold.

I was thinking about this last night and the sound and the way is varies makes me think of a really sharp exhaust leak. I am wondering if there is a AIR system leak (hose off or leaking) and one of the check valves is weak, allowing a leak at times. That's the only way that I can think of that allow an exhaust noise to come and go.
I'm down with a flu bug right now so going out and jacking up the car isn't going to happen today.

It doesn't sound like detonation but getting scan data does sound like a good idea. There may be one at work that I can borrow.

I appreciate the food for thought and this will all help me figure this out.
SharkPilot

[scottfab]

Originally Posted by SharkPilot

Scott, I can't access that link. It says it's not available for download.
The noise was the same hot and cold.

SharkPilot

Sorry to hear you're under the weather... sucks
Yeah, the link was working not not. Try going here:
http://scotfab.tripod.com/sound-library.html
If the text of the link gets stepped on try cutting and
pasting this link but remove the word JUNK in front of it.
JUNKhttp://scotfab.tripod.com/sound-library.html

once there click the play button.

[SharkPilot]

Thank you Scott.
That's not the sound I'm hearing which I suppose is a good thing.

I'm still thinking it sounded like an exhaust leak. That would explain how it changed with engine load. When I got home tonight I looked at what parts of the AIR system I can see from above and all look okay. I need to raise the car up and check down below. I'll see if one of those hoses on the bottom have come loose. That would make sense because the noise didn't seem to come from either side. I can't think of another place where an exhaust leak could come and go.
My theory is that the noise quit each time when the exhaust pressure went up when I gave it throttle to accelerate. That would be a leaking AIR check valve reseating. Does this make any sense?
This is one of those situations where I need it to make the noise again, at home preferably. On the other hand I hope I never hear it again.
I have listened with my stethiscope to each bank, touching the probe to certain cam cover bolts and everything sounds just like it always has. I make it a point to give it a listen once in a while, just to maje sure all is well in there.
Meanwhile the engine continues to run perfectly.

[SharkPilot]

I have an update on this:

I have been chasing an intermittant misfire and tracked it down to what I think is a bad plug wire on #8.
Pulled the plenum to replace the wires and found a missing secondary throttle screw on #7 cylinder. That wasn't a good feeling.
Prior to removing the plenum I was going to do a compression test. Then realized someone else had it, oops. So I removed the INJ fuses and cranked the engine. The rythem was very even, I know, not very scientific but it's what I could do at the time.
We looked in there with a boroscope and can see some evidence of it bouncing around on the piston but nothing that looks really bad. I just figured out how to use the mirror on the boroscope and may have spotted the screw stuck to the head. Not sure though and may need another pair of eyes again.
My question...
Is it possible to run a compression test with the plenum and fuel rails/injectors removed? I know I'll have to remove the fuel pump fuses and/or relays, but what else? I don't want to risk frying anything.

It looks like I'm going to be removing the engine and left cylinder head to fix this. I'd like to have compression results before I do that.

Thanks everyone.

[A26B]

Sounds like you found the cause of the noise.

Yes, it is possible to run a compression test with the fuel rails removed. Actually better, just remove the fuses to both fuel pumps.

Since you are going to be removing the engine, why not forego the compression test and do a leakdown test once the engine is on the stand. You will be turning the engine by hand and doing far less rotations of the crank & less exposure to further damage if the screw moves around during the process.

[Pete]

Here's a dumb question how many quarts do you put in it when doing an oil change?

Pete