  We report on measurements of low frequency noise in a single electron
transistor from a few Hz up to 10 MHz. Measurements were done for different
bias and gate voltages, which allows us to separate noise contributions from
different noise sources. We find a 1/f noise spectrum with two Lorentzians
superimposed. The cut-off frequency of one of the Lorentzians varies
systematically with the potential of the SET island. Our data is consistent
with two single-charge fluctuators situated close to the tunnel barrier. We
suggest that these are due to random charging of aluminum grains, each acting
as a single electron box with tunnel coupling to one of the leads and
capacitively coupled to the SET island. We are able to fit the data to our
model and extract parameters for the fluctuators.
