Might be actually a water line connected to the heater that is moving slightly as the pressure is released when the shower is turned on.
Sometimes water heaters make various sounds when water starts to flow often a "gurgling" sound and sometimes just "creaking" if the insulation on the outside is foam and the tank is contracting a very small amount as pressure is released.
If no leaking, I'd write if off as "normal". In RV's we live a lot closer to our Water Heaters so it's not unusual to notice any noises they might make quicker than in a house where the heater is in the basement or garage.
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2018 Flagstaff Micro Lite 25BDS
2004 Nissan Titan