Most likely one of the belts slipping. My alt/water pump belt did this on start-up and going hard round left hand corners. Check the tension of all your belts, and tighten them. Or maybe replace them if the're looking a bit worn. Hope this is of some help.
BOSS 5.4lt Quad cam V8 is mine
MkI BA XR8 with SS inductions, BMC air filter, Herrods Air Injection, Pacemaker extractors and cats back exhaust.
NQ Performance Ford Club Member