Hello
The floor in my downstairs bathroom feels hot. And also the faucet upstairs has a hiss like sound. And when I open my cold water ,I get hot water for 20-30 seconds then back to cold. I hope there is no leak under the house.thanks

"the floor in my downstairs bathroom feels hot"
How I hate to hear those words. You have a hot water leak under your slab that's costing you money as we speak.
It gets worse! Since water , like electricity, will seek the path of least resistance the actual leak may be feet away from the hot spot.
When we have this problem we call in outside help to locate and pinpoint the leak. Sleuth is the company that we use. They come in with sensitive listening devices and locate the leak, jackhammer up the slab and bag the dirt. The leak is then left exposed for the plumber to repair. The patch and tile replacement if the floor's tiled is up to you although the plumber usually puts the dirt back and floats in a cement patch before he leaves.
Sorry I couldn't give you better news. Good luck, Tom

Thank you, I was afraid of that.