Tips for chasing down engine noise??
 

I have a 96 993 3.6 Ltr with 29k miles. It has a engine noise that nobody seems to be able to put a finger on. I even went and had the motor rebuilt with new (used 7k) P+C's, arrow rods, valve guides, springs, DC21 cams etc. The noise is present from 1-2k rpms. It almost sound like a rod noise.

Until I know what the noise is it will drive me crazy just worrying that something bad is going to happen. Several track days before and after rebuild yet no change in the sound. When installing the GT oil cooler kit I did notice that I felt similar pulsing in the oil lines when running. Is it possible that the oil pump or drive gears would make that noise? We never did tear into the case.

I will also send the oil out to be checked for metal.

If you installed Arrow rods, you did "tear into the case" a little bit,...:) :) Hopefully, you carefully measured the rod journals and sized the rods and bearings for the proper crush & fitment.

Oil pumps usually don't make those kinds of noises.

Some detailed diagnostics will reveal the source of a rod knock. Block the throttle at the loudest RPM you hear the knock and with a helper, disconnect the plug leads (top & bottom) from each cylinder until you find the offending one. If they are all the same, you might be OK (assuming that you did all of the above when installing the rods).

I am comfortable that we set the rods up properly.

For some reason I can't get the motor to stay at the offending rpm. It wants to pull past it and not stay there. Maybe I will disconnect the ISV and see if that helps.

Thanks for the tips Steve!

Understood.

Using a long-handled screwdriver as a stethoscope, place the tip all over the engine (top & bottom) while its running to locate the source of the noise.

Sometimes these noises can masquerade as others, however rod bearings, piston pins, pistons touching heads, intermediate shaft bearings, and crank thrust bearings all have unique noise signatures so the stethoscope method is quite useful to locate and analyze what you are hearing. Rod issues are usually the loudest at low RPM without a load.

From the 3 mechanics that heard it they all feel its not a rod noise. Something about a double type knock. Given that bit of info and the fact we changed rods and bearings (old ones looked perfect) I have hope we can eliminate that as a source. Based on your description it sure does sound like rods given the RPM and no load.

One thought it was a piston slapping so I sourced some new (used 7k) P+C's even though my old ones looked good.

I used those electric ears to no avail. PS pump seemed to make the most noise. All in all listening to the motor through those things makes you think the whole thing will grenade any time. I will take another crack at it and see what I can find.

Double knocks can sometimes be caused by incorrect piston pin offsets but this case, all of the 3.6 Mahle pistons have the same offset so unless the pistons were installed upside down, thats likely not the issue.

LOL,...These engines make quite the symphonic mechanical racket when listening through those devices and it requires some extensive experience to decipher what you are listening to. :) :)

Is the noise better or worse with the clutch in? I have a 2004 Turbo that has the same noise, its the clutch plate!

The noise is with the clutch depressed. With it out there is a whole slew of gear rattle. That does not bother me cause I know what it is.

Because the noise is the same after changing the rods, rod bearings, P+C's (used 7k) I have to believe its something else. I am thinking of changing the rockers. When I took it apart for the mechanical conversion it looks like someone was already in there. The #5 rocker was installed in the # 2 cylinder. I converted to mechanical prior to rebuild trying to solve the noise. Someone said it maybe thrust noise from the rocker getting loaded and walking to the side. Whatever the case it appears that the mystery noise has been chased for years prior. I am thinking of testing the oil with blackstone and see what the test reveal.

Rocker arm/shaft noise can be isolated with judicious use of the stethoscope.

Were the intermediate shaft bearings replaced? Did anyone check thrust on that?

I would also check main bearing thrust, as well.

No on both counts.

Maybe I will take a road trip. :) I got nothing but time on my hands now.

Looks like you have some "homework" to do,.... :) :) :)