Loud foghorn sound when hot water faucet is turned on

Hello everyone, i've been searching the forums and found some posts with similar symptoms but not an exact match.

Over the last few days, every time the hot water faucet in the first floor bathroom is turned on, there is a loud foghorn like sound. Also,
1. The water flow from the faucet is diminished considerably when this happens.
2. If i turn on both the hot and cold water faucets in the sink at the same time, there is no sound.
3. So far, this has occured only in the mornings when we first use the bathroom. Problem disappears after repeated uses.
4. Opening any other faucet (hot or cold) anywhere else does not result in this sound.

The sound appears to be coming around the water heater area which happens to be situated very close to this bathroom.
We have 3 bathrooms in the house (1 on the 1st and 2 on the 2nd floor), haven't seen any other water flow issues.
From what i know, the water heater does have a pressure relieving valve on it. Don't see any expansion tank though. The water heater is set to "Warm".

Any ideas? We just moved into this place 3 months ago and I'd love to get to the bottom of this irritating issue.

BTW, this is my first post and i'm really a rookie when it comes to plumbing

Thanks

Loose washer.

Did you mean loose washer in the faucet? This is a cartridge type, 2 handle faucet. I could be wrong but I'm thinking washers are only found in the compression type faucet.

You may also want to check the shutoff under the sink and make sure it is open all the way.

Sounds like a loose washer is chattering or fluttering in the water flow. Could be in the angle stops if it's not at the faucet.

Or, a chunk of debris bouncing around...

If you have a Pressure Reducing Valve outside close to the house.If someone has made a wrong adjustment to it it will also vibrate through the plumbing system making that noise.

I really haven't heard that from a prv adjustment...
Failed maybe...

No if you have someone that does not know haw to adjust one and has it almost shut down it will make that noise I used to love it when homeowners try to get more pressure and end up the opposite

Thanks Terry, that did it! The shutoff valve under the sink was not open all the way. Uggh, i should have checked this first.
Appreciate all the responses.

Mine does the same thing for the same reason. I refuse to fix it cause it pisses the wife off. When you've been married a long time little things like that make your day.

Same issue different twist

Same loud fog horn, when turning hot water knob in bathroom sink. Sink is original (house built in 50s), couple of plumbers types said the assembly or washer is not manufactured any more, have no clue where to get that part. Tried replacing the sink, I was quoted $700 for replacing plus buying sink (shocking!!!). So now in MN winter we cannot turn hot water on specially at night, even our cats curse who ever trned that knob on anyway..please suggest...sink pic attached

sink

That looks like an old American Standard sink, and the parts that should make the sound are generic washers. But if it is a worn stem, that should also be available, but you would have to take it to the store to compare it to pictures in a catalog so it could be ordered.