Check this thread
https://www.talkmorgan.com/ubbthreads.php/topics/802035/clonking-brakes#Post802035I had this exact issue as noted in the above thread. For me it was due to the wire wheels being painted and the paint was enough to make you think the spinners were tight but after a while would reveal themselves as not so tight. Sorted this with the help of MWS who supply the wire wheels to Morgan. Removing some of the paint around the point where the spinner touches and then whacking the spinners on properly (using a spinner saver - for deep dish wheels)
You can check if it is this simply. Jack up the back corners one at a time and see if you can rotate the wheel a fraction backwards and forwards - with the handbrake on of course - if you can it's very likely loose spinners. As also mentioned some members have had the "pack it with grease" fix - mine were packed with grease but still clonked.
You'd be amazed how much of a thump you need to give the spinners to properly tighten them - then go for a mile or two - then thump then again.
Obviously make sure when first tightening the spinners, the wheel should be off the ground so the wheel is sitting properly against the hub. Lower the car, thump again, Drive the car. Thump again. If you get the spinner saver you need the deep dish one from MWS
