Yes the lyrics are disgusting, but they are just there for entertainment. Ever watch a horror movie? Or play a violent video game? Not to mention all the books written like that. It's no different here.
No, they are not "just making noises". They know what they are doing. However, I will give you that it's not easy to discern at first. I had to "get used to" Death Metal before enjoying it, and sure enough that's not for everybody, so I don't expect everybody to enjoy it.
Here are two examples with videos so you can see that it's not just random:
http://youtube.com/watch?v=QOb6JSQd-Qw
http://youtube.com/watch?v=QVJxCLcU6Wc
First one doesn't have lyrics, the 2nd one has some NSFW lyrics, but aren't "disgusting" as the regular DM ones are. It's a song about the prison system and the way justice favors the offender over the victim.
What I do hate though is Black Metal. They celebrate crappiness and take themselves seriously. It's just not my cup of tea.
http://youtube.com/watch?v=1WcsV7hQ1V4
That's not a song, but satire of Black Metal.