Knocking sound after take off

XJ40 1992 Daimler with working Self Levelling System SLS.
Occasionally after taking off and after approx 20 to 100 meters there is a mechanical series of knocks coming from the lower right side of the car. Wife says it comes from front and I say it comes from the rear…
5 to 8 knocks during approx 2 to 3 seconds. The series of knocks are not repeated during drive, only occasional occurs during the first 20 to 100 meters. Never after red-light stop but occasional after parking.
Can anyone please advice on solving this problem.