Wim,
a couple of things. First, check the hand brake cable length is correctly adjusted on the cable that comes from the lever (not the one that goes across the car). If it is incorrectly adjusted it can hook up on the propshaft and as soon as you move it snags and reapplies the hand brake. When you move the car the opposite way the handbrake releases. The setting is in thevTriumph repair manual. Also, make sure that the noise you hear is not the brake shoes binding on the drums and then releasing.
On my diff, one of the Allen bolts that hold the beaings into the diff had come loose and was catcking on the prop shaft bolts (to the point where it got bent and I didn’t notice it till the diff was removed).
