We know that the primary source of interference at HF is atmospheric noise. Thus, all else being equal, the strength of heard noise is a fair indicator of RX system performance (such as in adjusting an antenna tuner for maximum noise while receiving, in lieu of transmitting and measuring SWR), since the noise is received from all over and is not local interference which it would be useful to remove.
Suppose that we leave the frequency, antenna tuner, antenna orientation, etc. unchanged, but observe a change in received noise power. Does this reliably indicate a corresponding change in propagation conditions, or does the absolute level of atmospheric noise change independent of propagation?
In other words, if (I'm not making any contacts and) my radio is quieter than it was an hour ago, is it time to change bands?