[Eljaro]

If one of the caliper pistons stick slightly, even a runout of only 0.004" may be too much and cause chatter on light braking. When applying force the piston will move and chatter will disappear. Jaguar states maximum runout to be 0.006". I am working on front brakes of an E type V12 Jag which had bad front wheel pulsing and one disk had 0.006" the other .003" runout.
If caliper pistons move freely they can absorb that runout and you will not notice. Old calipers with slightly corroded pistons or brittle rubber seals will not move fully under light pressure and get the vibration through to the chassis and you will notice.