Strange Noise from Underneath

[Tattybogle]

Gentlemen,

Something to exercise your little grey cells!

My 1932 RN is fitted with late Girling axles front and rear and a 4-speed box with Hardy Spicer propshaft.

Lately I have noticed a strange noise from under the car that seems to only occur when the engine is pulling hard, say up a hill, often in 3rd gear. It sounds like a muffled dull knock, as if the floor is being struck with a hammer from underneath.

It doesn't always happen, but when it does the timing between the sounds can be one second or as much as five seconds.  I usually back off when it occurs because I don't know what it is nor what damage may be caused. The car otherwise drives normally although the CWP sings at 30 - 40 mph on light throttle only, never on the overrun or accelerating.

The gearbox and rear axle have been rebuilt with all new bearings.

I think the propshaft and joints are OK, but I haven't pulled anything apart yet.  

Your collective thoughts will be greatly appreciated.  Thank you!

[John Mason]

Does this happen when you accelerate hard and the torque tube is slack on the adjustment causing it to bang on its adjustable fixing.

John Mason.

[Chris KC]

Can you feel it or only hear it?

Worn universal joints rattle, I don't think you'd mistake them.

[Tattybogle]

Thanks for both contributions. I THINK I can feel it as well as hear it, but I have always been sensitive to mechanical irregularities and cannot be sure.
If I didn't know better, I'd suggest it was a tooth jumping in the diff, but I am sure it cannot be that as it would happen more often and not be just the one tooth. I've never heard of that happening anyway.

[Chris KC]

Inclined to agree with John but very hard to say without hearing it.

[Reckless Rat]

Perhaps, given that you say the rear axle has been rebuilt that perhaps the torque tube mount hasn't been correctly set up after reassembly. When you say you can feel & hear the knock, is it coming from under the floor behind the seats or from the bulkhead? The only other thing I'm thinking about is the front crossmember/radius arms ball joint, the rivets mounting the ball to the chassis can become loose. One last possibility is a road spring knocking on the underneath of the body - you'll be able to see if this is happening if you jack the car up, remove a rear wheel and look to see whether the leaf clamp has been contacting the tub - there'll be a mark. (ask me how I know that - anyone got a pair of Nobbies they don't want?)

[dickie65]

Take the cover from transmission tunnel by the handbrake and go for a drive with a passenger to look down at the propshaft beside the handbrake.
Its not unknown for a propshaft to tap the handbrake mechanism.
You should also be able to take the grease point cover from above the torque tube and do the same.