What's this fitting called?

Hello all Whenever i turn on my bathroom faucet OR when after flush, water is filling in the tank... I hear a loud water running noise. It is loud and can be heard upstairs or downstairs. This noise does not come when i turn on my kitchen faucet or bathroom shower. Just BOTH bathroom flush does this and bathroom faucets creates this sound. Water pressure in my house is around 55 psi. The sound does not feel like water hanmer sound. I do not feel any vibration. This sound is like water is rushing through the pipes but it is just too loud. This sound stops as soon as the faucet is closed or flush tank is full. Please help here.

House ca. 1958 I noticed some rusty boils appearing on the outside of my cast iron pipe which is the main waste leaving the house. Now, I know relatively nothing about this stuff, and for all I know, it could have been there for some time, but I just now noticed it, and there was some rusty residue running down the pipe from the boils. I attached four pictures. I have been living in this house for just over four years. I did some research and see some people say it's better when it's a vertical pipe and not to worry, while others are saying start stock piling money as this will be an issue in the future. These were from other websites and forums I have read, not about my own issue. Any thoughts? Should I do anything myself, call a plumber to change it out, or just let it go until something potentially happens.....which is scary in of itself. Thank you!