[R-390] Questions about receiver sensitivity

[Anthony Casorso]

I am making some measurements of the sensitivity of my R-390a and I have some questions. Hopefully the collective wisdom can guide my thinking in the right direction.
The basic question is whether there is a fixed relationship between AM and CW sensitivity. In other words, if I measure the CW sensitivity at some bandwidth, is it really possible to infer what the AM sensitivity should be at another bandwidth and modulation percentage? I am measuring AM sensitivity as modulation-on vs modulation off.
That leads me to ask if it is possible for a receiver to have excellent CW sensitivity but lousy AM sensitivity (modulation on-off)? If so, what kinds of things could cause this?
Last, and this feels like a dumb question, why does the receiver output noise rise when I give it an unmodulated carrier in AM mode with the AGC off? What is the source of the noise? I am assuming that my signal generator is not the issue (Boonton 102C) but I suppose anything is possible.
Thanks in advance,
Tony