Shimming recessed convector

I have a one-pipe steam system in my home, and all the heating sources are all convectors recessed in the wall behind a decorative cover. The convectors are encased in a thin metal housing. There’s a single convector that consistently experiences water hammer even after having the system serviced by the local steam pros. (I didn’t have them look at this convector because the cover was painted shut.) I was finally able to get the cover off to check level and found that it’s pitch every so slightly away from the pipe side. I want to shim it up a little, but the convector won’t seem to budge even though it doesn’t look like it’s fastened. Anyone have any tips? I’ve posted some pictures, but please let me know if other pics would be helpful.