Agree, probably the outlet check valve. Since units vary greatly on the location of the HWH, you may have to check around to see how easy or difficult it is to get at. Mine is under the kitchen sink and not too bad to get at if you can stand on your head! I carry a spare one, and a long reach 1" socket to remove it. Again, they may not all be the same, so check yours out first. It may just be scale in the valve, but the plastic internal parts are prone to breaking. You may be able to find an all brass one.
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