SoRtEd!! 😃
Found three of the nuts loose on the nearside of the manifold. Whiped the heatsheild off and Re-Torqe'd them all up and it sounds sweet again! took less than five minutes. If your engine sounds slightly tappity on startup It should be the first thing you check! However:-
WARNING
DO NOT tighten them without a torque wrench!! 12nm or 9 ft lb (108 in lb) is not very tight at all and probably the reason they come loose. If you go at them hammer and fist your probably gonna snap the quite thin studs off in the head and then your really in the sh!t. Drilling small steel studs out of an aluminium head is black magic. If you havent got a torque wrench then dont use any more than one finger on the end of your spanner to nip them up and you should be ok.