We estimate the ultimate noise of Gunn oscillators in the absence of 1/

noise. The basic noise source considered is thermal or Johnson noise augmented by intervalley noise of carriers hopping between the high and low mobility bands. For example, for Q
ext= 10
2and P
out=10
-1W we estimate δf
rms≈ 1-2 Hz, and AM noise relative to the carrier of -156 dD, both measured in 1-kHz bandwidths.