knock knock!! driveshaft? propshaft? gearbox?

[al 350i]

i seemed to have gained a new noise when driving along at low speeds in a high gear (30mph, 4th) there seems to be a knocking coming from the middle to rear of the car, seems to happen when applying a small amount of throttle, like its taking up some slack. i have jacked the car up at the back and put it on axle stands then run the car with it in 3rd,4th and 5th at tickover and can hear a constant metalic grumble or knocking, it seems to be worse the higher the gear, if i increase the revs the noise disappears. the rear diff bushes were replaced about 8,ooo miles ago and i have checked the bolts and bushes for any play and them seem ok, the gearbox was replaced at 36,000 (61,500 at present) so like to think it is ok. the driveshafts feel ok with no slack or splits on the gaiters, not sure on the rear diff i have just replaced the oil and the old stuff did look a little tried and the casing does seem to have leaked a little oil for somewhere!

before i start taking everything apart has anyone got any suggestion or test i can do to try and pinpoint the problem? i'm thinking it could be the uj's on the prop so was going to go crazy with the grease gun to see if that quietens it down but can only access the rear grease nipple, how can i get to the one at the gearbox end?

[GreenV8S]

With the handbrake on, very gently engage the clutch alternately in forward and reverse gears and see if you can reproduce the clonk. If you can, then you should (with care) be able to track down where it's coming from.

[VPower]

Not sure if this helps, but just had mine up on the ramps today to move the exhaust line-up away from the chassis rails and cross support.

Drove to LE Mans with the most awfaul noise that I was convinced was either the gearbox or the timning chain about to explode!
My mate grabbed the near side exhaust and lifeted it a bit and all went quite!

Hoping that moving hte exhuast has cured it, but I suspect that the internals of my nice new ACT quite system may have come loose???

[Steve_T]

Check diff mounting bushes (common cause of clunks) and if the exhaust is rubbing the chassis (common cause of 'orrible noises). I found I needed to drop the exhaust a bit to clear the prop shaft heat shield as ACT use inline silencers in the big bore system.

[jesfirth]

if the clonk occurs when backing off the throttle it is usually the top diff mount loose or cracked. Might also want to check the gearbox mounts they are close to the exhausts and often crack due to heat

[al 350i]

found split gaiter on drivers side driveshaft so hoping that will be the cause of the problem. does anyone know if it best to just replace the cj or buy a whole new driveshaft? is the driveshaft shared with anything else?

[jesfirth]

clever trevor sell uprated ones for £175