Ummm, if your lifters were hitting the valve cover (!) you'd sure have real problems. In fact, if they were, it would be pretty weird. As it is highly unlikely, especially if the engine is running at all, that this is what is really wrong. Maybe you should get a second opinion! If a lifter flew out of its position, there would be some god awful noise, along with extraordinarily rough running, as one (or more) cylinders wouldn't be firing and/or exhausting (yikes!) AND your camshaft would be very unhappy! Check out the thread "End of my 4.0", as he had some similar sounds..... Can you describe any more details? Engine? Year? Anything?
[size=1][b][color=purple]2000 XJ SE I-6
5-Speed NV3550 Stick
Rusty's Air Tube w/K&N Filter
2" Lift with 30 X 9.50 BFG AT KO's[/size][/b][/color]