Diagnosing a hum

[chalitoona]

I got a 1970 Karmann Ghia as a graduation gift and I do need some help diagnosing a hum that doesn't seem to be problematic but kind of a nuisance. Once I pass 30 miles per hour it used to make a hum, kind of a rubbing sound. This sound is in the rear. Naturally I thought the tires were rubbing. So I got them balanced and the guy told me they were way off, so I had wishful thinking hoping that was the fix to the issue. (I was wrong). I drove it around and thought that was the fix to it, as the hum went away. But the noise decided to return once i passed the 35, 40 ish region. Now the thing that confuses me is its not there when I let off the gas, even if I am going the speed that it usually makes it at the noise wont appear when I am off the gas. The hum seems to come out when I accelerate slightly, not full pedal to the metal accelerating, just getting up to speed. The weird part is the hum isn't as intense when that is being done, instead the hum is much much louder when I slightly press the gas to maintain the speed I am going, which would be roughy around 40 if you need to know. Im not talking pushing it down much at all, it is when I am very slightly on it to keep me in the same speed. Feathering the gas you could say. I've taken it to a few mechanics and VW nerds, but no one seems to figure out what it is so I am hoping someone will come save my sanity. I have gotten transmission rebuilt, the wheels balanced, the bearings packed, carb rebuilt, and essentially a complete engine service.

[wagen19]

It´s hard to say from distance, but I suggest to check the 36 mm nuts for the rear brake drums. Pull the pins off and check the torque with a torque wrench. If the nuts are loose, check also the drums for "play" on the shafts. The drums can wear out, if the nuts are not properly tightend. With bad luck, you need new drums, if not more.
Good luck!

[22manybugs]

The hum starts at about 30-40 mph, does the hum go away at higher speeds?

Typically a hum is bearings. You said the bearings were repacked and transmission was rebuilt. That should take care of any bearings... BUT: the last two times I bought a rebuilt transmission (two different shops), the transmission had something wrong out of the box. One made noise in all gears, and one made noise in second gear. Each time I had to take the transmission back for the shop to fix. On the rear wheel bearings, did someone take the bearings out, clean them, and inspect them? Just repacking them will not help if the bearings are badly pitted/cracked/worn. (Removing the rear wheel bearings is a lot of work. Honestly, by the time I remove rear wheel bearings, I replace the bearings so I know they are good and new and I'll get years of use from them.)

It sounds more like wheel bearings, or possibly loose 36mm nut like wagen19 said, instead of the transmission. It's hard to tell from the description.

You might lift the rear end with sturdy jack stands, start the car, put in gear and let the wheels spin off the ground. Then you can listen for the hum while the car is stationary. That could help to localize the hum to left, right, or center. You could also see if tires are rubbing or wobbling as they spin. If you do this, please be careful, use sturdy jack stands (do not use just a jack to hold up the car), make sure the car is stable on the jackstands, and best to have someone with you.
_________________
Lon

1969 Karmann Ghia convertible
1971 Westfalia camper

[chalitoona]

Alright, Thanks you guys will be doing that when I get back from vacation asap. As far as the bearings go I wasn’t told about any replacement so might as well do that and hope, and I’ll for sure tighten that bolt to spec. I know my right rear is to spec and the noise doesn’t seem to be apparent on that side

As far as the noise being or going away when I’m above that I honestly haven’t much a clue, I’ve driven it on the highway twice but above 50 I don’t have the best hearing with the wind. But I’m sure it is

What would the transmission sound like if it was jacked? Would it be like a crunch or grind? What I have sounds like a hum or even a higher pitched rumble. Obviously I can’t get you an exact sound but when I get back and drive I’ll see if I can get a recording.

For jacking it up and listening, obviously it would have to be crazily sturdy. Would you recommend I just spin it in like first gear and basically have it spinning at a low speed or try to get it higher. I know seems like a dumb question but hey, you never know till ya ask…

[22manybugs]

In my case, the transmission issues were a hum or grrrr noise. The 2nd gear issue was a loud grrr that varied with engine speed, but only in 2nd gear. The other one was a grrr that got louder over several months and was louder as I went up in gears, to the point that it was unbearably loud in 4th at freeway speeds.

A crunchy sound that happens continually from the transmission could be a broken tooth or broken synchro, that's not the sound you described. CV joints might also make a hum or crunchy sound.

If you jack up the car and run it, start in 1st and see if you hear noise. Shift to the next gear if you don't hear it. I have used the stepped cog on the carburetor to hold the engine at a fast idle so the wheels spin a little faster. Since you will not have all the driving and wind noise, you may hear the noise at a low speed, but if the noise does not start until 30-40 mph, you might have to go to 3rd or 4th gear to hear it.

I use a stethoscope with a long metal tip (available at auto parts stores) to help listen for bearing noises. Touch the tip to the left & right rear wheel bearing housings (both near the wheel and away from the wheel to listen to the two bearings in each housing) while the wheels are turning, a noisy bearing is usually obvious with the stethoscope.
_________________
Lon

1969 Karmann Ghia convertible
1971 Westfalia camper

[chalitoona]

Thanks so much, I will be seeing if I can figure it out when I get home. I wish there was a better way to describe the noise over chat, but I’ll see if I can upload a sort of video so you can hear the exact noise.

Thanks!