[dcw]

Which engine does your GLT have?

And do you have any drop in performance of any kind to go with the noise?

Is the noise there all the time - ie when idling and driving, or just one or the other?

It could be any number of things; as has been said I believe the oil pumps were pretty reliable, but injectors might be faulty I guess... You appear to have been able to rule out pure engine related things like the valves and camshaft - what about the cambelt and its pullies?

Is the noise definitely coming from the engine itself?

Or from any of the other belt/pulley combinations - alternator, water pump, power steering pump, a/c pump (if yours has it)?

David