1998 Crown Victoria Valve Chatter?

1998 Ford Crown Victoria LX 4.6L Use 89 Octane BP, Shell, Chevron gas. When the engine is under stress(acceleration or going up incline) the engine sounds like the valves are chattering. I can increase the rpm's and the chattering goes away. Had the same problem with 95 Thunderbird 4.6 and have heard other ford cars going down the road with this problem. Air leak? Don't really want to use higher octane due to gas prices and possible engine damage. Also, in the thunderbird, i used a bottle of octane boost and the next day a valve spring broke. Was the octane boost a possible cause?