In the bathroom when you turn on the hot water to a certain point it
sounds like a bottle rocket about to explode. Turn it more and it stops.
Air in the lines? Some thing else? It just started a few days ago. Is it
a problem?

Also DW noticed leak under sink in kitchen. I turned the handle a bit
and it wasn't shut off so I made sure it was and now no leaks there. We
do use a portable dishwasher so there is some back pressure daily.

Are the 2 connected in any way? Should I ask city water about air in
line or is that problem?

Albee Kuminova wrote:

In the bathroom when you turn on the hot water to a certain point it
sounds like a bottle rocket about to explode. Turn it more and it stops.
Air in the lines? Some thing else? It just started a few days ago. Is it
a problem?

Also DW noticed leak under sink in kitchen. I turned the handle a bit
and it wasn't shut off so I made sure it was and now no leaks there. We
do use a portable dishwasher so there is some back pressure daily.

Are the 2 connected in any way? Should I ask city water about air in
line or is that problem?

Not air. The faucet stem threads may be worn out or the washer
may be loose, either one causing a vibration.

Jim