I have a 1973 Beetle 1300 SP swing axle which has a rumble in the rear end which occurs only when I take my foot off the accellerator and let the car run on. It occurs at all speeds. It goes away when I apply the brakes even slightly. What is it likely to be?

Like a light rythmic bump-bump-bump???

I suggest rear torsion bar grommet

No. It doesn't bang or bump or thump although it is rythmic. It goes, grumble, rumble, grumble, rumble, grumble, rumble etc., at all speeds. If I have any amount of power on, ranging from full throttle to light throttle, or apply the brakes even lightly, it stops. Does anyone have any ideas as to what it might be?

Maybe a wheel bearing, looseness in the brakes??

Wheel bearings or gearbox are the likely cause. Let's see what others think...

could it be a gearbox mount?sounds like wheel bearing but that shouldnt stop when you apply the brakes.

wheel bearing or loose axle nut

jack up the rear of the car and check for push and pull play or side to side play in the wheel.

You say it is coming from the rear?,
Check the front wheel bearings, noise can travel, I'm not joking!!

I spoke to several people about the rumble in my gear box and they all said that I have a problem with bearings on the main shaft.

The late great John Muir (he of the Compleat Idiot's VW Guide) said that I should put a tube of Molybdenum disulphide in the gearbox. I gave it an oil change to 85W-140 Hypoid oil, and a tube of Moly-di. It's a bit quieter. The rumble is still there, but it's not as intrusive as it was.

I found a disturbing amount of metal shavings on the magnetic drain plug so I can only drive it carefully, get another gearbox and do a swap if I need to.

Bad Luck....

I have been using additives in gearboxes since the early 70s....
Put them in every car I've had....
Never ever had a gearbox problem....
I'm using NULON nowdays but I used to use MolyD
years ago...
Its great stuff...
Don't forget when You grease Your beetle to use Lithium based Moly grease -[castrol LMM] in Your front end...
wheel bearings etc.... its the best....

I hope Your gearbox keeps going but it doesn't sound to good...

cheers

Lee

did you jack up your car and check for play?

if it goes away when u apply the brakes id say its because the brakes steady the drum as it spins stopping the noise.

Thanks for your comments and suggestions. The noise goes away when I apply the brakes and also when I change down into, say, 3rd gear until the engine braking effect ceases, then the noise returns, so I think that it is a gearbox problem not in the brakes.

I'll check the bakes on the morrow.

Update on the rumble, or is that the ex=rumble?

When I back out of the car port and the gearbox is cold it feels like the hand brake is slighlty on. There is thick gooey cold oil in the gearbox.

After a good long run down the freeway today at a conservative 90 klicks, the rumble has almost completely gone. Now it sounds like a bit of crown wheel and pinion noise, so I may have solved the problem with new oil and molybdenum disulphide.

I know that I have a problem, so I'll take it easy and see how long it lasts as a solution to the problem.