Confusion with Bode plot in LTspice when using a voltage regulator
I believe that the reference level for LTspice ac plots is that 0 dB = 1 V. So, it looks like your ac signal is very, very small. Keep in mind that this is a simulation, not real life, so there are round-off errors that can accumulate and look like very small noise signals.

EDIT: Your simulation provides an input voltage of 1V for the regulator. An LM340T-12 is not going to function properly with that small of an input voltage, you need at least 14V to make this device work. I think your simulation results are probably bogus.