With the van being 1995 it will have a Carver Cascade water heater, most likely a Carver gas fire ,an Electrolux fridge.
If the "tapping" sound is from an oblong unit with a handwheel on the top and is in the cold water pipe as it enters the van and has two wires attached to it, that is the water pressure switch, which needs adjusting to get it to work correctly.
With a van of that age there are likely problems with the water heater inlet as there is a non return valve which gets stuck and it would need a new fitting. Trying to use the hot water system if the tank is not full of water will cause serious damage to the heater.
If you have just bought the van, and presumably have no idea when, if ever, it has been serviced, I strongly recommend getting a mobile engineer to service it and explain how all the items work .
It will save you a lot of trouble in the long run, as well as knowing everything is working correctly for your safety.