Spyder_Man

Hey folks,

I've got another possible issue cropping up in my 993. The other day while driving on the freeway, I had to come to a near full stop on a down hill slope due to traffic backup. No incident occurred, but I was shocked to suddenly hear some vibrational noise that I hadn't heard before.

I've been listening for that same hum since and here is what I've observed. The hum seems to be a lower pitch vibration that corresponded to the rev of the engine. It is most noticeable on a downhill include with the engine braking (I.e using the engine to slow from 3-4k rpm to below 2-3k rpm). Braking on the downhill made it more noticeable. I think it is occurring at higher RPMs too and possibly on level ground but the normal roar of the engine with the throttle engaged makes it much harder to hear that lower frequency.

It seems to me like drive train vibrations are more directly being transmitted to the cabin that before and more noticeably when the car is on a downward incline with weight shifting forward under braking, so I'm wondering do these symptoms suggest a failing motor mount?

What other possibilities could it be? I'll check out my alternator fan to make sure it isn't rubbing, but I suspect that would sound differently anyhow.

Thanks in advance!

Spyder_Man

I will add I haven't noticed any additional movement or vibration in the shifter **** which I'm guessing would be symptomatic of a failed transmission mount. Also I have had y engine mounts replaced with RS mounts in the last year, so I know those are fine.

nine9six

I would suspect the same as you...Trans mount member or motor mounts. How old are your RS mm? They do go bad like anything else.
EDIT:
I see you posted again while I was responding. Do you track your car?

OverBoosted28

Easiest thing to do, is get her up on stands, pull front, trans covers and go thru everything. Use a pry bar (carefully) to see if you can get movement where you shouldn't. If something shifts, while your prying, is will surely move while driving. Jacking the bottom of case/engine, will show you if MM have failed (movement). Could be a number of things that transmit noise vibration from other areas, during different conditions. No vibration from front end/steering? I know you recently did the susp. triple check everything. Good luck and let us know what ya find.

Spyder_Man

Thanks for the tips. The steering, suspension and motor mounts were all overhauled during the last year and it's felt solid since the install so I'm suspecting the original parts with nearly 170k miles of use are to blame. I haven't tracked the car at all, yet...

The noise is definitely not coming from the steering if anything it sounds like it's coming from the rear and/or below the chassis. Hence my motor mount suspicion.

I'm going to see if my local wrench can make some time to put the car on the lift and pry at the transmission mount a bit as OB suggests. His shop is 5 minutes from my office so it is far easier for me to have him look at it quickly.

Spyder_Man

One more question, if I do need a new transmission mount what do go with? OEM standard mount (possibly with FF inserts), OEM RS mount (to complement my RS mms), or something aftermarket?

pp000830

Tires can me noisy and sound like gear noise that becomes much louder on deceleration and roll to stop under braking. I think it has to do with how the surface of the tread blocks wear and in some cases exhibit cuping on the face of the blocks. Some tires are more prone to this. I had the issue with RE50 tires at one time. The ones I use now do not seem to have this issue.

Andy

OverBoosted28

This is what I had looked at in the past, but it's only for 2WD cars. Quality item though

Spyder_Man

The noise was there again when I drove home. It definitely only happens when in gear and is most noticeable when heading downhill with the foot off the gas and the engine braking kicking in. I appreciate the tip about tread wear noise, but this is definitely not the tires.

Tomorrow AM I'm having it looked at by my wrench. Hopefully it will be sorted early next week. I found an OEM RS mount for under $150 from a reputable source (Pelican and Sunset are both asking more than $300), so I jumped on it.

I'm pretty certain its the transmission mount, but that paranoid little voice in the back of my mind keeps thinking "What if it's the alternator fan failing?" I don't think that noise would depend on the transmission being engaged to make the noise though...

Spyder_Man

So I drive the 993 to the mechanic this morning and the rattles are the worst I've heard it yet. Then with him in the car we drive around on the same roads at the same speeds and no rattle. He's still going to take a look at it.

I'm wondering if it is worse when the engine oil an transmission fluid are still warming up.

pp000830

Transmissions have gears cut to minimize noise pulses as the turning forces transfer from tooth to tooth on the gears in the transmission. Also teeth, I suspect, embed themselves over time and become quieter. When one switches from acceleration to down shifting to slow down, the forces are switched in the gear sets to the opposite side of each tooth on the gears that engage. The back side of each tooth probably sounds louder than under acceleration as the teeth do not run nearly as often under load as the accelerating side of each tooth face and, I would think are a good bit louder. This sort of noise is normal. Now the idea that this noise is transmitted to the cabin to a greater extent due to a failed mount causing a metal contact from the transmission to the body seems reasonable. The mount would have to be very warn for this to happen. Like rubber parts braking up. I guess the issue is how loud is the noise as these cars are >15 years old and for a number of reasons, not requiring repair, are not as quiet as when new.

Andy

nine9six

Taking another stab based upon add'l info...CV joints?

Spyder_Man

CV joints are a possibility given the mileage. I sure hope that isn't the case based on pricing!

Anyone have an opinion regarding whether its better to just replace the inner CV joints with a kit and pay the labor or swap out the whole drive axle assembly for far less labor. How much labor time would I be saving with a whole axle swap?

OverBoosted28

For the most part, anytime you take your car to a "stealer" or a wrench, you can't reproduce the problem, "noise", vibration or condition that is an issue. "Murphy". Let us know whatcha find.

nine9six

I'd rather it be my CV's than failing trans mount.

I'm sure someone here will correct me if wrong; but isn't the latter of the two require an engine drop, or at least a partial release of the motor? If it's the trans mount, I think Rennline offers a more robust, bolt in unit.

Bottom line and stating the obvious here; your gonna need to confirm the culprit, prior to moving fwd.

Curious to hear what your wrench says...Pls keep us posted