Bongo Fury

The injection pumps have an advance and retard sytem that times injection acording to engine load/revs. If this goes out of kilter it will cause a horrible noise but it sounds like an old truck engine - not marbles. One faulty injector can make a banging noise so it could be that, but I would expect to hear it at light load as well as heavy load. It might be worth checking engine mounts and exhaust/cooling pipes for knocking. Have a good check underneath for anything that might clang under load.

I've just had this reply to posting your question on another forum.

I have had a similar noise in a petrol Opel Manta auto and the torque convertor had come loose on the flywheel. managed to tighten it up one at a time through an inspection hole. Perfect after that.

That would sound very logical to me.

I think you can get access to the T.C. bolts through the plate bolted to the gearbox which holds the exhaust bracket on.