You have some sort of grounding problem either with the home's electrical wiring, or the external display, or possibly an issue between the external display and some other external device which also has its own power source.
You want to make sure all external devices which connect to the electrical outlet are all on the same circuit. Sometimes an two electrical outlets in a room could be on different circuits and it is also possible for an electrical outlet can be wired to two different circuits (the latter is rare).
Try using a grounded AC cord (aka Power Extension Cable) with the laptop's charger.....it will have a third prong for the dedicated ground. Sometimes this will help if the home's electrical wiring has an issue. Unfortunately it seems some regions do not use a dedicated third prong for ground. If the powered devices are connected to a power surge strip, then maybe that power strip is bad. The Duckhead adapter which is provided by default does not include a dedicated ground.