I think I know what you are talking about and it is pretty normal. It is just the slop amongst gears when at lower rpm's there is not a constant tension on the mesh. I have it in my GTS and 944 and used to have in Corvettes with Borg Warner T-10 trannies, plus a few others.

 

Did/ does it in the '88SE (LSD) using Castrol Synthetic, ?? Blue/Green stuff i can't remember the name of the 911 crowd use in the 915 box, and now with Valvoline Synthetic

Did/ does it in the '90GT (PSD) using Mobil Synthetic, and now with Valvoline Synthetic

Did it in Paul UKkids '87 with no LSD - no idea what oil. Would be interesting to know if it has stopped with his new MMs.

In my book it's not gear oil dependant and not LSD/PSD/open diff related.

 

Thanks for the replies everyone. I'll just assume it's a normal occurrence when the rpms are too low for the gear.

I did the motor mounts 2 1/2 years ago and I don't think it made a difference. On the other hand, the motor mounts did make a difference on the smoothness of shifting.

It seems like it may occur more frequently when the transmission is good and hot (after higher rpm runs), but I'm not sure. Maybe that's why the 93+ cars get manual transmission oil coolers?

 

My transmission is currently drained and I'm waiting on a few bottles of Redline 75w-90NS to arrive. After this gear oil change, I'll pay closer attention to see if there is any difference.

 

Hi,

I have a '90 GT with 50K miles on it. I have the exact same phenomenon. A couple of my friends with GT's and a GTS have the same thing.

Actually one of them has had his GT since 20K miles, and the sound was there already at that time.

Our investigations indicate that it is not a transmission sound, but the exhaust rattling in these situations it is driven by the torque applied at low RPM in 1st and 2nd, in particular. The rattling may very well be there in other situations, but not to the extent that it makes this noise. We have been confirmed by several experienced Porsche specialized shop owners.


Ragnar

 

Sounds like "ping" or like a 70's Chevy AIR pump diverter valve, but coming from the wrong end of the car. Noise gets transfered a bit through the torque tube, but is from the rear. I dislike guys speculating without direct experience, but it seems like our layshafts and mainshafts are long and poorly supported. It sounds like the gear sets are rattling because they're flopping around in the case! They resonate like tuning forks apparently since the noise is very rpm dependent along with load. A string vibrates based on it's tension, specific gravity, and (I think I rem.) Young's modulus, etc. Our '86 posi 5-sp does the rattle at the same time as reported - 1 000. to 1 500 under modest load.

 

The new oil didn't make a noticeable difference. Noise is still there. Next time, I think I'll try a different oil.

Where would it be rattling at? I've had mine off several times (at least) and even changed the rubber supports. The noise doesn't sound like exhaust vibration to me. It sounds more like gears that need a thicker oil, or dino oil, or something along those lines.

 

Correction, it is not a rattling or a rattling sound, but a resonance sound. The exhaust tubing does not rattle against anything, it is a resonance sound within the exhaust tubing itself.

Sorry for the mistake.

I have not done anything to get rid of the sound, but i believe that it can be silenced by fixing or dampening the exhaust tubing somehow.

Again, if it actually was a sound generated by the transmission, why does it appear consistently on so many cars, with a wide range of service conditions, age and mileage ? It appears in the same conditions, does not dissappear over time and it does not develop into a transmission faliure or anything alike. It just stays there ! Consistent decade after decade (Hey, that is rare !)

It appears to more likely be a design issue than a matter of wear / condition / lubrication of the transmission.


Ragnar

 

My 91 GT also has this sound, i think it is the same sound; at low rpm's in 1. or 2., 1000 - 1500 rpms while accelerating, perhaps worse in turns, sounds like an accelerating diesel truck.

Seems more like the sound comes from the front, around the clutch or torque tube?
My thoughts were release bearing or torque tube?

 

My 90GT has the same noise, 1400-1600 rpm, accelerating, foot off the clutch. Sounds/feels like it's coming from the cbellhousing area. I figured it was throwout bearing or somesuch, which hasn't been changed in 50K miles, but then I don't know what I'm talking about.

 

Exactly!
I am thinking this is something that is worn out or broken, but if everyone experience the same noise it may be a design issue.
Tempting to replace the clutch including all bearings just to find out, too bad the clutch is still holding up..

 

The sound I am referring to is definitely not coming from the clutch area. I previously replaced all of the clutch parts, except the intermediate plate, as part of the regular maintenance. It is coming from further back in the the rear axle/transmission area. It could possibly be some kind of torque tube vibration, but the sound does not correlate to any vibration throughout the car (like motor mounts do).

After regreasing the clutch intermediate shaft, resetting the intermediate plate adjustment fingers, replacing the rear coupler bushings, and bleeding the slave and master cylinders, my clutch and transmission shifts very smooth now in all gears. I can even shift into 1st at 20-25mph.

The accelerating diesel noise is the only "problem". I am still running Redline gear oil 75w-90, but eventually it will be changed out to a non-synthetic straight 90w if I can find one. The diesel noise almost sounds like the gears are vibrating between a certain rpm and it occurs more frequently on hot days.

 

I'm not hugely experienced in manual transmissions but noone in the thread seems to have mentioned transmission mounts?

I haven't changed mine yet, so don't have a baseline for comparison (I'll be doing it after I get around to motor mounts/pan gasket), but from various old threads around here I understand that cars with 100k+ on them typically have saggy transmission mounts which would reduce the vibration dampening at the rear end.

Does the vibration cease above the rev threshold, or fade away as revs rise? (resonant frequencies will just disappear as soon as the wavelength goes outside the resonator's cavity dimensions).

 

ahh, good question. They just disappear in my case.

 

Same for me, the sound disappears when i reach a certain RPM. Actually the sounds only occurs in a very narrow RPM band in 1st and 2nd. Appears to be related to the torque applied.

I can not say whether it occurs and disappears at exactly the same RPM's in both 1st and 2nd.

I am replacing my Motor Mounts, and when i take for a spin i will try to determine whether the sound has changed.

In my case the sound comes from forward of to just below the drivers position. Certainly not from the rear.


Ragnar