First, look at the water meter to verify that the water is actually running. If it is, and you aren't using any, you have a leak. Also, check to verify that the house doesn't have a hot water recirculation system installed; if it does, that water sound may be normal when the pump is running. Older tanks are likely to have deposits in the bottom, and both normal operation and recirculation might sound louder than when new.
Many houses in Texas are built on a slab, and the pipes may run underneath the slab. See if you can find any spots that seem warmer than others, or are damp (may be hard to see or feel because of floor coverings). Also look outside to see if the grass is greener for unknown reasons anywhere along the foundation.
If you can't find the leak point(s), the next path is either just rerun the hot water through the house (maybe in the attic), or find a leak detector service to pinpoint the leak location, dig up the slab and repair. Note, if you have one leak now, there may be more later, so this can get very disruptive and expensive.