Transmission sound starts again as well as a slight slip?

Recently had my transmission replaced by a individual mechanic. While driving it home it started making a knocking sound from the flexplate/ torque converter area. Called mechanic he said the flex plate was cracked so I bought a new flex plate and he installed it for me. Now less than a month and maybe 200 miles later and the sound starts again as well as a slight slip in the transmission. I check the fluid, and find it's not even on the dipstick. So I put fluid in and started paying attention to it daily as I have no other way to work. The fluid is leaking and the sound I am hearing when it was a cracked flex plate would get a little quieter and sometimes almost stop when I was driving now as the rpms increase so does the speed of the sound. The mechanic now refuses to even look at it gave me a third of my money back and quit taking my calls. I am pretty good with cars once I understand what I need to do but I've only ever replaced manual transmissions. Help me figure out what or how to diagnose and fix the issue please.