Rear End "Growls"

Someone wrote - I have a '72 Beetle... I'm trying to diagnose a grind or growl that happens as I'm coasting. The sound seems to be coming from the rear of the car. If I'm accelerating there is no growl, but as soon as I let up on the accelerator, it growls pretty loudly. I've replaced the transaxle fluid, and when I took the drain plug out, I didn't see an abundant amount of metal shavings. So I'm thinking that it could be in the brakes or something.

Rob responded - It's good that you didn't find any metal shavings, but doesn't mean there isn't wear in there.

First try to find the source of the growling sound -

• Does the noise follow the engine rpm, or the car speed (which would make it a drive train problem)?
• When coasting (in gear, foot off accelerator) put your foot on the clutch - does the noise disappear? If it does, then reappears when you engage the clutch again, the noise is almost certainly coming from the gearbox.

(I have a similar noise in reverse gear only when I take my foot off the accelerator (slowing down in reverse). My '68 Bug's gearbox has unknown miles on it but is PROBABLY original - 140,000 miles.)

This kind of noise usually indicates that the bearings in the gearbox are worn. It might hold together for quite a while, or might let go very soon.

• Make sure you have the right tranny oil - any decent brand 80w90 or 85w90 hypoid gear oil works well (you can also use synthetic 75/90 gear oil). The critical part is the 90 higher number - this sets the viscosity for the oil when it's warm. An 80w90 is a little thinner when cold than an 85w90, so (the 80w90) will give you smoother gear changes when the oil is cold. If you like, you can also try adding a tube of teflon (TPFE) for gearboxes. Don't use the additive for engine oil, it has to be suitable for gearboxes (here in Australia you can buy Nulon for gearboxes, but you could try looking for a Slick product to suit in the USA. Teflon in the gearbox DOES smooth it down, definitely helps smooth worn synchros, and might help worn bearings.
• If the noise is the same with the clutch in or out it could be in the rear axle bearings or CV joints. The CV joints usually make a clicking sound when they are worn out, but dry wheel bearings can certainly make a continuous grinding sound, which changes with the road speed. There's a procedure on our web site for regreasing both the CV joints (messiest job you can do on a VW) and the wheel bearings.
• Binding brakes usually make a scraping sort of sound, and the brake drums will get quite hot after just a few miles if they are binding, so that's easy to test for (remove the hub cap and place your hand on the wheel centre where it's attached to the drum). If the sound only happens as you touch the brakes, it's a good chance the shoes have worn out and need replacing. One indicator of worn brakes is the hand brake coming up high before it works (it needs to move the shoes more before they engages the drums).

The guy wrote back - The growl is definitely related to car speed (not engine speed), and is not affected by engaging or disengaging the clutch. I guess its probably the wheel bearings from what you have suggested, it's not much of a clicking sound, but a grinding. So maybe that's it. The strange thing is that its not constant, I would have thought that dry wheel bearings would grind constantly? I would rather replace wheel bearings than work on the transaxle...so i guess its sort of a relief.

Rob responded - Your further description (mainly the intermittent nature of the noise) makes me wonder if you have one of the big axle nuts a little loose and the brake drum is moving, making the noise intermittent.

To test for this, jack each rear wheel off the ground in turn and grab top and bottom and push/pull in-out. If there is movement on the axle you've probably found the problem. If that's it, it might just need the axle nuts tightened (217ftlbs and don't forget to put the split pins back through the nut), or it might need a replacement axle if the splines have been damaged.

It could still be the wheel bearings too - worn bearings can also cause some sideways movement in the axle through the hub assembly, which would produce an intermittent sound.

The guy wrote - I will definitely check the nut...that sounds like that could possible by the case. What size is that nut...I think i might have to use a pipe wrench huh? I'll check the wheel bearings as well, just to see.

When I was towing my car from my storage place (tow rope, not a tow bar) I rode the brakes a little too hard, and they got hotter than the sun. :-) Grease was coming out of the front wheel caps, so I repacked the front wheel bearings, but didn't do the rear ones, assuming that they didn't get near as hot as the fronts...maybe they did.

Dave wrote to ask - I was wondering whether you had resolved the "growling" problem and what it turned out to be.

The guy wrote - Yes, the problem was resolved (sort of), as it turns out, the axle nut was loose, so the growls have gone away, but I'm still worried about the bearings. Is there supposed to be some play when I push (up) on the drums? It's not much, maybe a millimeter or two. The side that was "growling" definitely had more play, but got better when I tightened the nut. Now both sides have equal amount of play. What is your advice? I will definitely change the bearings, but I'm just curious how it's supposed to be...

Dave responded - I'm not a rear wheel bearing expert, but a mm or two sounds excessive to me. Are you absolutely sure you have at least 217 ft-lbs of torque on the axle nut? This is the minimum requirement -- it must be very tight to prevent the bearing from wobbling.

Rob wrote - 1mm movement is a little too much - it should be almost no movement at all.

So - two possibilities - the wheel bearings are worn and need replacing, or else the two ring spacers on each axle (used to position the bearing inner races) have been inserted the wrong way round and need to be removed and reinstalled. On each side there are two ring spacers and each has one chamfered edge, and these MUST be orientated the right way or the stack of spacers, bearings and brake drum can not be tightened properly by the 36mm nut (incorrect spacer orientation MIGHT have contributed the the nut coming loose).

The gearbox-side spacer must be the one with the inner chamfered edge and the brake-drum side spacer must have the outer chamfered edge, and both chamfers must face the gearbox. This probably wont make much sense unless you take it all apart, but you'll see what I mean when you look at them. The space between the two bearings also has a spacer - this is called a tube spacer since that's what it is (longer than the ring spacers.) The bearings must be shiny bright and not have any pit marks or be "blued" (which indicates they've been overheated.) If the outer races are shiny bright and unmarked they can be left in place, but otherwise they are an interference fit and must be driven out (the gearbox side outer race is held in the brake assembly is held in place with a large circlip.)

The space between the bearings must be completely filled with grease since the bearings are not sealed - the grease filling acts as a reserviour for the bearings.