oscillon

Hi All,

I have searched in vein for some info about this strange and somewhat intermittent chattering sound that seems to originate in the engine of my very stock 1996 993 C2.

A bit about the car:

105k miles and runs very strong. SAI ports and full top end done at 75k. very recent (100 miles ago) tune up including all plugs, plug wires and all 4 O2 sensors. Rebuilt distributor (all bearing and the belt). New alternator and fan housing. I am a service nut and try very hard to make sure everything is in top operating condition. The DMF and the clutch were both replaced at about 65k. The clutch seems fine and doesn't slip or engage in a strange point in the pedal range.

Aside from needing a tune up I actually had hoped that the sound I was hearing was related in some way to older plugs etc. I have a lift, so I spent some time under the car looking for anything (shrouds etc) that might be loose and susceptible to vibrations. I had the mufflers off and the exhaust seems fine overall.

The problem:

On my way home from work after driving in some mild city traffic, meaning that the car is fully up to operating temperature (and perhaps a bit hot due to stop and go conditions). I can, on demand produce a sound that sounds a lot like valve train chatter at a very specific RPM regardless of gear choice. If I hold the engine speed at just below 2000 rpm, right at roughly 1800-1900 rpm in either 2nd or 3rd gear, the rattle sound which I would describe as an RPM coupled chatter is very noticeable. It sort of reminds me of the sound a car would make if you tried to start in third and the engine or perhaps the clutch chatters like crazy. The sound only occurs at this small range of RPM's.

I don't know if this could be one of the hydraulic lifters, but I don't get any chatter or ticking while the engine is cold. In fact, the noise only comes when the temperature is a little on the high side. If I downshift and raise the engine revs, the noise is gone. I'm not sure if it's a valve issue or perhaps a chain problem or maybe even a clutch spring sound.

If anyone has a lead, or can point to something obvious that I could inspect, I'm all ears.

Thanks in advance.
Dan

pp000830

I would think a fully engaged clutch even one with springs in it isn't going to chatter. On release maybe.
Could be engine tin rattling or exhaust components external or internal
If you still use the engine under-body tray it could be the source as well.

us a rubber mallet and a wooden stick and tap on thinks with the engine off and see if anything rattles. Include the mufflers as their internals can be loose. If it was the catalytic converter I would suspect it would not be performing up to spec and you would have a check engine light.

axl911

Is it engine pinging or knocking? Accelerating a car from a low rpm puts lots of load on the engine.

P-daddy

Are you sure the clutch was replaced with a DMF /clutch set up? It sounds exactly like the chatter you get with a LFW/clutch when you lug the engine in 2nd or 3rd in that same rpm range.

pp000830

Interesting, a worn out dual mass flywheel can rattle a bit. This is a very very uncommon issue. - IF your car seems to idle a bit too rough and you have been experiencing:
 The check engine light and;
 A reoccurring non-cylinder or;
 Multiple named cylinder “emission relevant misfire OBDC code.
AND you have addressed the other less invasive sources of this such as:
 Run the quick, unplug-the-primary-distributor, belt test,
 Verifying there is not a vacuum leak on the engine,
 Cleaning the idle valve,
 Replacing the distributor caps and rotors and,
 Addressed any cylinder specific reoccurring misfire codes, wires, plugs, and injector (done after the items above and a clearing of the codes with the misfire code still returning).
You may have a problem with the dual mass flywheel (DMF).
The reason for the DMF in the 993 is to address the engine‘s ignition-induced rotational speed irregularity cause of vibration in the driveline. At certain speeds ignition timing matches the natural vibrations of the driveline amplifying the vibration causing transmission rattle and body boom.
I am told the litmus test for a failing DMF is to use the PSTII (Porsche tool), put the car in the air and with the car in gear, read the speed sensor directly. I'm told that it will rapidly slow and speed up out of sync with the engine RPM. Also LuK offers a DMF testing tool that physically tests the flywheel once the transmission is removed. If the flywheel needs to be replaced light weight non-dual mass flywheels are widely discussed on this board as a replacement option.

OverBoosted28

Mechanics stethoscope, and search till you pinpoint area. Could be as simple as a piece of tin rattling (exhaust heat shield). From what I gather, lifter clatter, can be pretty loud. Good luck

user 723923490

+1 on exhaust clamps and fasteners. I had to replace pretty much everything at 80K including the cat bracket & clamps. My heat shields were also rattling around before I replaced them.
Cheers

bcameron59

This.

HPNer

^^^^
Yep, that's what mine sounds like too!

Leander

I would double-check the fan bearings. Fan could be contacting the shroud.

oscillon

Hey All.

Thanks for the collective wisdom. I went back to the service records and it does have a DMF that was replaced 20K miles ago. I also just put a new fan, housing and bearings as my housing had deteriorated which led to a loose alternator.

I presume it's a heat shield, but I may dive in to see if all of the valve springs and lifters look alright. I guess the lifters would be the next order of business to replace/upgrade.

Cheers.

More to come at some point for anyone who is interested.

ppv3

Have similar noise at 1800 RPM 1991 C2 triptronic loud rattle like tin runs noise free after 1800

Endoman

Something is loose or broken and has a natural harmonic frequency induced by the engine at those revs. Exhaust components, heat sheilds, engine tin, even disc dust covers. I had a baffle in the mid silencer of my 944 come loose, a bit unlikely in a 993. Forget tappets, if it was a clutch/flwheel component it would stop or change when the clutch was operated.

IainM

ppv3

i was wrong----the rattle occurs at 1800 rpm in neutral and 1st gear-rattle stops at 2000 and higher--when down shifting it occurs again at 1800 rpm