My AC has never worked from day one because the compressor was bad, so I finally replaced it today. My problem is that everytime I turn the AC on - the catalytic converter starts making this loud humming sound. Is this normal or is there a problem? What affects does the AC unit have on the converter to cause this noise? Thanks in advance.

 

the only thing it could be is extra resonation being transmitted through the exhaust. Make sure the cat is attached tightly and that the shield above it isn't vibrating.