Rearend Noise

[fromadig91]

So I just got back my 04 Cobra a few days ago from the shop and it has been raining so didn't drive it much. Yesterday I took her out and it seems around 2400-2600 I get this drone coming from the rear end (so my dad says) I thought it was the transmission but after his explanation it seems feasible. When I had the rear diff cover replaced I also had some IRS bushings installed. My mechanic said he changed the "pitch" with these bushings and my car does feel a bit different, but then again after driving a lancer for 2 months I feel I can't make a 100% sound decision. The work I had done was the following

-More durable diff cover installed (at least twice as thick)
-IRS bushings
-New FRPP driveshaft installed (snapped the old one's yolk at the rear diff)
-reinstalled stock mid-pipe (shouldn't matter)

I'm trying to figure out if the "stiffing" up of the rear end is causing me to hear and feel more vibrations or not, also I have never driven my car in < 50 degree weather but I gave it substantial time to warm up, and even once the trans is all warmed up I still hear this noise.

This noise is only audible at 3rd and 4th gear (mainly) and its under load, so if I take my foot off the gas its gone, this is also why I feel its my gears in the diff but I simply don't know anymore at this point

Maybe the gears did need to be replaced after the blowout? How would I know for sure?

Suggestions? Thoughts?

 

[trinity_gt]

Stiffer bushings can definitely lead to more gear noise in the car. Anyone that's replaced their OE shifter with an aftermarket one realizes just how noisy gears can be. This is one of those NVH (noise/vibration/harshness) things OE engineers spend so much time trying to balance with handling, responsiveness, durability and so on when tuning and selecting things like bushings.

Having said this, you might want to prod your mechanic to more clearly state what he meant by "pitch." Did he alter the pinion angle (the angle the pinion axis makes with the driveshaft)? What sort of bushings were they?

With very stiff or solid bushings you may hear quite a bit of road, tire and gear noise but that doesn't mean you don't have underlying issues. Is the final drive gearset original? If not, there's a chance the gears were not properly set up, resulting in, say, insufficient pinion bearing preload, incorrect backlash etc.

When the driveshaft was replaced, was the flange on the rear end touched at all? If the flange was removed and replaced (e.g. because it was damaged when the yoke broke) the pinion preload may not have been restored when it was put back together.

 

[fromadig91]

By pitch he did mean the pinion angle, I was sure to be clear about this, and the fact that you mention "With very stiff or solid bushings you may hear quite a bit of rad, tire and gear noise" is interesting and makes sense to me, because they were aluminum IRS differential bushings that were installed (Kenny Brown)

I am not sure on the driveshaft part thou, and will be sure to ask him.

yes, they are the standard gears in the car, nothing after market, the way it came from the factory.

Thanks for the quick response, you always seem to have excellent technical advice.

 

[fast97gt]

is the cover the type that has the girdle style adjusters in it, if so the other problems may be imminante, because they could have over tightened the cap stiffners and now the you hav a bearing noise in the rearend

 

[trinity_gt]

By pitch he did mean the pinion angle

Do you know why he altered the pinion angle? Is the car lowered or used for drag racing a lot? Or is this just a consequence of going to the KB bushings?

Typically, going to solid bushings is done in an effort to eliminate wheel hop which can be caused by rubber bushings that have some give. With solid aluminum bushings, you're definitely going to hear more noise in the car. If the driveshaft flange wasn't touched, you can probably chalk the extra noise up to "normal" for solid metal bushings.

BTW, a bit of give in complex mechanical systems isn't a bad thing unless you're a competitive racer (e.g. F1 or something...) Without flexibility provided by compliant bushings, you're going to be feeding a good deal more shock loads directly into the housing which, on a daily driver, may not be ideal. Of course if the car is a racer then you're willing to trade off NVH and possibly reliability to prevent wheelhop which will damage the rear.

In the case of live axle cars, it's well known that the LCAs have very compliant front bushings. Part of this comes from the necessity for something to give as the control arms all work different arcs as the axle housing -- a rigid body -- moves relative to another rigid body -- the body/chassis. If one replaces the LCAs with "drag" units with stiff, urethane front bushings, its very possible that the lack of "give" can actually rip the upper control arm mounts off the floor and/or fatigue the floor sheetmetal into cracking. This sort of thing can be repaired and drag racers are willing to do it but for daily drivers, such damage is unwanted from "upgrades".

I am not sure on the driveshaft part thou, and will be sure to ask him.

yes, they are the standard gears in the car, nothing after market, the way it came from the factory.

If the flange had to be removed for some reason, the pinion nut would had to have been removed too. If he didn't check the pinion preload before removing it and failed to re-set it afterwards, that may be a source of pinion bearing noise. Worthwhile asking, anyway...