Our bathroom sink never seemed to have this trouble before and not sure when it started.
The hot water will flow fine as long as the water flowing out is not hot, but still "warming up". Once it gets so HOT water is flowing, there is a sudden loss of pressure and an awful noise and vibration.
If cold and hot are run together, it does not do this.

What could the problem be?

TIA!
Scottcrew

Hi Scott,
One or two handled faucet? Brand name? How old? Back to you, Tom

Two handled, brand unknown, approx 5-8 yrs old.

Two handled, brand unknown, approx 5-8 yrs old.

Sounds like you have some crud under the seat or a broken washer that's vibrating as the water goes past it. Here's what you have in the hot water side, (see image). Turn the water off to the hot side and open up the faucet.
If the washer's cracked or in pieces then replace it. Now place a pan over the open valve and turn the water back on to flush out any crud that may still be in there. Reassemble and test. Better now?

Haven't tried it, yet, it may be a day or 2.
But, my question is why won't it do this when the water is still cold or lukewarm coming out, but only once the water gets hot?
It won't do it, also, when I have both the cold and hot handles open? Another thing is that the hot handle has to be fully open or almost fully open before it will do it, either.
So why would a washer cause that?

Haven't tried it, yet, it may be a day or 2.
But, my question is why won't it do this when the water is still cold or lukewarm coming out, but only once the water gets hot?
It won't do it, also, when I have both the cold and hot handles open? Another thing is that the hot handle has to be fully open or almost fully open before it will do it, either.
So why would a washer cause that?

I don't know! Sounds like the pressure change has something to do with it. That's why you have to open iit up. Good luck. Tom