mysterious knock - xjs front end

[jim meehan]

Now engine problems have been resolved, another gremlin has appeared.

A knock / bump, on driver's side; not related to steering position or road surface. Can be a few just taps or a loud series of worrying bangs. At loudest, sound comes from just in front of pedals. Two garages have checked it out. Not steering, brakes, anti roll links or suspension problems - for a tenner my local garage took it out and said it could be springs resting in their seats moving about a bit - but could find no loose bolts; for another sixty quid, a specialist sat it on ramp and told me it's a broken cat in the exhaust - but it's not and he never road tested it. To my untrained ear, it sounds like another lost spanner behind the wheel arch.

Any ideas?