928S 5 speed, Humming/Droning from rear

MARCL2377@gmail · 05:21 PM

Hello.. So I have a 1985 928 5 speed. 72k miles. Was running and driving perfectly but was due for a new timing belt.. Took to the shop. Had it done. Driving home and now it has a constant hum/drone from the rear. Took it back, put it on lift.. Listened with a stethoscope while running at 50-60 mph. Sure enough it's coming from the rear. Not wheel bearings or axles or torque tube.. Doesn't matter if I'm accelerating, decelerating or turning.. Doesn't matter what gear. always there. Car still performs perfectly, but is now too annoying to drive. Just started out of no where.. Was silent before.. This is a well maintained 72k car.. Never tracked or flogged.. Thoughts? Thank in advance!! Marc

stepson · 06-20-2016, 05:45 PM

Wheel bearing, axles and torque tube are the only things moving in the rear end besides gears in diff. Did you check gear oil levels in the diff?

stepson · 06-20-2016, 05:47 PM

Unless.....UNLESS it's a fuel pump drone. You might check the fuel pump up against something setting up a harmonic.

Danglerb · 06-20-2016, 10:19 PM

If the drone is constant, doesn't change if you change gears, that suggests the issue isn't forward of the transmission as the speed of the TT etc change with gear changes. Any change with it in neutral?

If it comes and goes depending on speed, that doesn't sound like fuel pump. OTOH if its constant independent of speed that sounds like fuel pump.

Gear noise usually changes with load, but not always.

I would record the sound and post a link and see if anybody recognizes it.

Has anything been changed in the sound deadening that might have blocked the sound in the past, but now allow you to hear it?

5l32v4c · 5l32v4c's Garage

Quote:

Originally Posted by stepson

Unless.....UNLESS it's a fuel pump drone. You might check the fuel pump up against something setting up a harmonic.

x2 - if it's not speed-related, this could be the culprit. Also, make sure your wife or mother-in-law is not sitting in the back as this can be the source an annoying droning sound as well.

stepson · 06-21-2016, 08:51 AM

Quote:

Originally Posted by 5l32v4c

x2 - make sure your wife or mother-in-law is not sitting in the back as this can be the source an annoying droning sound as well.

LOL. Your wife obviously does not see your posts on this forum.

JK McDonald · JK McDonald's Garage

Hey Marc - Just a guess but when the shop changed out your timing belt you might ask if they dug down under the spare tire and disconnected the battery. Perhaps they unintentionally left out the sound deadening material on the rear deck when they put everything back together ? I was in a hurry one time to drive Miss Purdy and left the rear deck bare to let the carpet dry after a good shampooing. It sounded like I was dragging a can of rocks around.

After trying several types of material - to muffle any hint of the drone from my fuel pumps (Miss Purdy/Lady in Red) I finally installed a series of the old timey thick pads that secretaries use to put under their noisy typewriters and printers. When everyone changed over to laser printers many offices threw these things out by the truck load. They are just the right size to fit side by side under the carpet and make things (pert-near) whisper quiet.

Good Luck, Michael

danb928 · danb928's Garage

Any chance that the mechanic or any other employee from the shop that did the timing belt may have taken it for a "test drive" and treated the drivetrain differently than you do. Unfortunate thought, but a similar situation happened to a friend of mine with a Corvette.

MikeB7 · 09-24-2016, 06:13 AM

EXACT same situation suddenly.

What did this turn out to be?

Mal81 · Mal81's Garage

You can get enough of that thick stress pad type floor foam to quiet the entire car for $7 at Harbor Freight. Great stuff. BTW, I tested it for fire. It WILL burn, but not rapidly.

-M

xwcgt · 10-31-2016, 04:18 PM

wheel bearing.. if so , it would change tone with side to side loading. (doing S turns for example at high speed)

MikeB7 · 11-01-2016, 11:41 AM

Exactly, this one's a puzzler because:
-Side to side makes no difference so presumably not wheel bearings.
-Varies with road speed only (not engine rpm), so post transmission.
-Does NOT vary with load ie acceleration/decelaration makes NO difference in sound. That would seem to rule out the usual differential suspects.

In talking with the original poster, the one possibility raised that seems a possibility is pinion input shaft bearing. Again, I would still think that noise would vary if you let off the throttle (and unload it), but maybe not?

Danglerb · 11-02-2016, 06:43 AM

"Maybe" an expert listening and inspecting it could pick the troublemaker, but only maybe. The torque tube rigidly connects the front and back so noises are very hard to locate.

Some ideas:

Noise was always there and some plug or bit of insulation is missing or deteriorated.

Check the fluid in Trans and diff, amount, condition, any residue. Refill with a good product.

Drop the rear end and open it up rebuidling as you go.

Drive it to an event where a bunch of people can listen to it.

Jim Chambers · 11-02-2016, 10:29 AM

Quote:

Originally Posted by Mal81

You can get enough of that thick stress pad type floor foam to quiet the entire car for $7 at Harbor Freight. Great stuff. BTW, I tested it for fire. It WILL burn, but not rapidly.

-M

Can you provide more detail on the product and where you installed it? I am trying to mask the noise from my wheel well fuel pump.

xwcgt · 11-03-2016, 01:51 PM

put car on jacks and run the car with no load on the bearings... bearing noise will still be shown, but you can put it in 5th gear , 2000rpm and get out of the car and listen.

then, do the test in say 3-4th gear 3000rpm and then ride the brakes (putting some load on the wheels) that willl put a load on the transmission and that might make the sound you are looking to detect. you have to narrow it down so you can determine the source.

Mal81 · Mal81's Garage

Quote:

Originally Posted by Jim Chambers

Can you provide more detail on the product and where you installed it? I am trying to mask the noise from my wheel well fuel pump.

HI Jim,

Sure, here's the description.
Anti-Fatigue Foam Mat Set 4 Pc
It often goes on sale for $6.99 in the monthly coupons and during their parking lot sales.

I have it slid under all my carpets, and it has significantly reduced outside noise coming in to the interior.

Jim Chambers · 11-03-2016, 06:13 PM

Thank you. I will do the same.

JK McDonald · JK McDonald's Garage

Hey Marc - Did you ever figure out what had been the sourse of the new noise with your car ? Was the garage that changed out your timing belt any assistance ? Like some other members, I try to keep an on going "Did Not Know" (DNK) file current with any service and repair information that solves any unusual problems on one of our 928's.

Michael