No one responded with a like model so I'll toss this in. Look in the vicinity of your fresh water tank. Usually very close to it. Can you tack the sound down? Turn water on and walk around coach...the sound should lead you right to it. Do you have a plumbing diagram? You can download it from Thor website or call Customer Service and they will email you a copy.
2018 THOR Chateau 35SF
Two Labs, Bugsie & Blondie