As JohnnyB good stated, you have a piping leak. It may be leaking even when not using the water pump, it is is just more aggravated when the pump runs.
If you hear the water pump run a few seconds every now and then when the pump is turned on, and you are not using water in the camper, then there is a leak bleeding the pressure off. The leak bleeds off the pressure and causes the pump to burp to build it back up.
This leak should be address sooner then later. Water damage in a camper is painful to correct.
Let us know if you need more help
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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