The LM399 is way more noisy than a reasonable quality OP.
But a good (selected) LM399 has around 4uVpp 0.1 - 10 Hz noise.
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For a voltage reference the noise of the LM399 is not that bad, but for comparison the 0.1-10 Hz noise for an OP07 is around 0.4 µV. The 1/f noise cross over for the LM399 is also not that different from precision OPs, so looking at different frequency bands would not change that much on the ratio. The LM399 is more noisy than the OP07 at any reasonable frequency. This probably still works for the 741 and LM358.
It takes a really not suitable CMOS OP like the MCP6002 to get something like 6 µV_pp noise.
Also remember that noise from different sources adds like squares. So it would not be a row of 10 OP07 configured as a buffer to give a 4 µV noise, but 100.
OPs noise can be a problem if at a position where the noise is amplified.
One reason for the rather high noise is the 30 V range used to measure 7 V. This range already uses a divider at the input - a divider by 10 and the 3 V if you are lucky, but it could be even divide by 100 and the 300 mV range.